>> NARRATOR: Tonight on Frontline...

When Edward Snowden exposed the NSA's secret mass surveillance programs... >> The public needs to decide whether these programs are right or wrong.

>> NARRATOR: ...defenders of the NSA said the spying was necessary, and they produced an example.

>> David Headley.

>> David Headley.

>> NARRATOR: David Coleman Headley, an American, recruited by Pakistani terrorists to help plan the most spectacular terror attack since 9/11.

>> So many innocent people lost their lives.

>> NARRATOR: But was his arrest really an intelligence success?

>> Nobody detected him, nobody stopped him, lots and lots of people died.

>> NARRATOR: ProPublica reporter Sebastian Rotella unearths new details about Headley's shadowy life.

>> It's a draft memoir.

This is the first time it's been made public.

>> NARRATOR: And evidence of how high-tech spies accessed the terrorists' communications but still didn't stop the attack.

>> What role did the program play in stopping Headley?

>> None.

>> NARRATOR: Tonight, Frontline and ProPublica investigate an American terrorist.

>> SEBASTIAN ROTELLA: It was July 2009.

A man from America had come here to Copenhagen on business.

But first, some sightseeing.

>> First of all, he rents a bike, an ordinary bicycle.

And he has his video camera, and he rides with one hand on the handle and holding the video camera, riding around and taking pictures.

You know, it's such an unusual scene of someone biking along with a camera.

>> ROTELLA: These are his actual recordings as he narrated the images along the way.

>> ROTELLA: He might have been playing the part of a tourist, but his true intent was to case the city and this newspaper.

Jyllands-Posten.

Four years before, it had caused outrage across the Muslim world by publishing 12 cartoons of the Prophet Mohammed.

Now, the visitor's plan was to launch an armed siege that would bring it down.

His name was David Coleman Headley.

He was an American terrorist.

I came to Copenhagen two years later, retracing Headley's steps on the globe-spanning journey that had brought him here.

I'm doing a story here in Copenhagen about the David Headley case.

He was planning an attack in Denmark.

>> Yeah.

>> ROTELLA: I'm trying to reconstruct his movements and I was wondering if you guys remembered him?

>> Maybe I can get back... >> ROTELLA: He was ostensibly a businessman.

>> We are back in 2009... >> ROTELLA: There he is, David Headley, right here.

>> Headley, yeah.

>> ROTELLA: He needed to get access to the newspaper.

I'd been trying to make contact with someone from Jyllands-Posten who had dealt with Headley.

>> ROTELLA: Hi, this is Sebastian, I just got your email, how are you?

>> ROTELLA: Good, good.

I appreciate your getting back to me, I know you're busy.

That would be great at 4:00.

>> Okay, well, I'm a typical Danish blonde, curly hair.

Headley was acting very normal.

He told me he was a businessman, he was here for business.

He wanted to move his business to Denmark and wanted to buy space in the newspaper for advertisement.

I remember that he exchanged business cards with my colleague.

He didn't seem suspicious in any way.

We didn't have many walking in from the street wanting to... to buy space in the newspapers like that.

>> Headley was certainly someone who seems to be extremely well prepared.

He was using a cover and he went to some lengths to conduct that surveillance, but also in targets surrounding it as a fallback plan.

>> ROTELLA: He cased the area around the newspaper and videotaped nearby restaurants, a hotel, and the French embassy.

>> ROTELLA: He walked into Copenhagen's main train terminal, his camera recording every moment.

>> It was a very professional way to plan a terrorist attack.

He was doing reconnaissance and planning in Denmark, and that he was subsequently able to pass over part of the planning to terrorists in Pakistan.

>> ROTELLA: Headley's reconnaissance of Copenhagen was sponsored by a notorious terrorist named Ilyas Kashmiri.

At the time, he was reporting to Osama bin Laden.

Headley was working for Al Qaeda.

>> ROTELLA: It's very chilling, you know, because Headley gathers all this information-- the videotape, the notes-- and he meets with one of the most fearsome terrorists in the world, Ilyas Kashmiri.

And they have a detailed conversation about how this plot would go down.

Three or four attackers with automatic weapons who go in and take hostages, but add a wrinkle, which is the beheading of hostages.

Kashmiri says, "You shoot the hostages first.

"It makes it easier to behead them.

You behead them, and you throw the heads out the window."

>> It is without doubt the most serious plot that Denmark has had.

This is a symbolically powerful target.

This is it.

There is no other more powerful way which all Muslims-- jihadists or non-jihadists-- will understand.

You will be instantaneously the hero of the jihadist world.

>> ROTELLA: But what David Coleman Headley didn't know was that just days before, he'd been picked up on the radar of Western intelligence.

On this trip to Copenhagen, every step he made and every bicycle ride was being monitored.

>> We had a security service shadowing him on their own bikes.

>> ROTELLA: Soon Headley headed for home.

That's when American officials learned of his connection to Kashmiri, the Al Qaeda kingpin.

By the time he landed in Chicago, they had him under surveillance.

>> Well, certainly the concern is, is there a homeland plot here in the United States?

We received the authority to go up on Headley's communications real-time.

Anybody that came in contact with Headley became a target of ours.

>> And that's when it kind of grew into a larger picture of who David Headley really was.

>> ROTELLA: An American terrorist, age 49, living in an immigrant neighborhood on Chicago's north side with his wife and four children.

Over the next three months, they investigated: tapping his phone, monitoring his movements, digging into his background.

They learned that Headley was planning a trip to Pakistan and then Denmark.

>> It was cold and raining, Chicago, and we drove out to O'Hare Airport to make the arrest.

>> We had complete surveillance coverage of him while he was driving to O'Hare, and I had surveillance coverage on foot in O'Hare.

Once he arrived, he went to the ticket counter.

We let him get through security, and then we approach him.

>> I looked at him and said, "Are you Mr. David Headley?"

And he was a bit surprised, and he said, "Why, yes, I am."

My partner uttered the words, "You are under arrest."

>> So I mean I don't know, but I want some...

I mean, it doesn't matter what I want, but I'm saying, I'd like some busts to happen.

You know, I don't want to keep on...

I mean, I know you have plenty of evidence against me, but... >> ROTELLA: He immediately began to bargain, giving up details about his Denmark terror plot.

But there was more.

He confessed to another attack, this one bigger and deadly.

>> The largest impact was when he initially confessed to the interrogators that he had an active role in the Mumbai plot.

So many innocent people lost their lives because of this individual that I had just handcuffed.

>> Just hours ago, terrorists launched a brazen attack.

>> The Indian city of Mumbai is in chaos following a series of terrorist attacks.

>> Mumbai's been hit, and hit hard.

>> ROTELLA: Mumbai, ten months earlier, had been the scene of a horrific three-day terrorist siege.

(explosion) >> You heard a big blast right now inside the Taj Hotel.

>> ROTELLA: The world had watched as an icon of India was set ablaze and civilians-- Indians and Westerners-- were methodically gunned down.

>> They wanted anyone with British or American passports.

>> I could see someone lifting up a gun like that and firing.

>> The attackers were well armed and well prepared to launch what some here are calling India's 9/11.

>> ROTELLA: 166 people were killed.

Headley had played a key role in planning the attack.

>> You have any plans to get after these guys, or no plans yet?

>> ROTELLA: Now, facing a possible death sentence, he bargained.

>> There are some things we can't share with you as well.

>> ROTELLA: Confessing he'd worked for Pakistani intelligence and Islamic terror groups, Headley became a witness for U.S. prosecutors.

He was convicted and disappeared into a maximum security prison, almost forgotten.

For several years, I had investigated Headley's story, reporting for ProPublica and Frontline.

>> A secret surveillance program is collecting the telephone records of every single one of us.

>> A 29-year-old man who says he is the one who exposed... >> ROTELLA: That was behind me when suddenly in 2013, news broke.

>> He's Edward Snowden.

He worked for the National Security Agency.

>> He's a traitor who seriously endangered U.S. national security, or a hero defending privacy?

>> The public needs to decide whether these programs and policies are right or wrong, and I'm willing... >> ROTELLA: The Snowden revelations incited a debate over privacy and how invasive the government should be.

But the NSA said its surveillance programs, like the mass collection of phone records and email, helped stop terror attacks.

>> In order to find the needle, we needed the haystack.

>> There is no other way that we know of to connect the dots.

>> ROTELLA: In the debate between privacy and security... >> Secret programs approved by a secret court based on secret interpretations of the law.

>> ROTELLA: ...the NSA's defenders rushed forward an example.

>> David Headley.

>> David Headley.

>> One of those involved perpetrators of the Mumbai bombing in India, David Headley.

>> Headley was working on a plot to bomb a Danish newspaper office that had published the cartoon depictions of the prophet Mohammed.

>> ROTELLA: I was surprised when David Coleman Headley was identified as a success story in the war on terror.

My reporting up to that point had shown a broad intelligence failure, an American terrorist who had operated for years without being caught.

>> To quote the Deputy Attorney General in justification for this program, if you're looking for the needle in the haystack, you have to have the entire haystack to look through.

>> ROTELLA: Now the NSA was saying he was proof that their newly revealed electronic surveillance systems worked.

Soon after Snowden's revelations, our team got access to his leaked documents, revealing startling new details about the work of spy agencies in the Mumbai case.

Together with my own investigation of Headley, it raised questions: Why wasn't Headley stopped sooner?

And could Western intelligence have prevented the attacks on Mumbai?

It meant I was back on the trail of David Coleman Headley, trying to understand what the NSA might have known.

I had started in Chicago, where Headley lived with his Pakistani wife and four children on the North Side near an old friend, a man named Tahawwur Rana.

>> ROTELLA: Mrs. Rana, how are you?

You could tell me what do you know about how they met?

Headley and Rana became close in the 1970s growing up outside of Islamabad.

>> This is Dave.

>> ROTELLA: David Coleman Headley.

>> Mm-hmm.

>> ROTELLA: The Ranas considered Headley family.

>> He was very, very nice.

He called me sister and he said, "You're my sister."

And that was a really good thing.

>> ROTELLA: For years, the Ranas had been operating an immigration company.

It specialized in getting foreign professionals U.S. worker visas.

>> ROTELLA: Was your husband working here then?

>> My husband, he particularly doesn't have any desk... >> ROTELLA: Headley used the business as a front while he was plotting the attacks in Mumbai and Denmark.

>> ROTELLA: Did David Coleman Headley ever come to this office?

>> Yes, but he was not working in this office.

He was working in a Mumbai office.

>> ROTELLA: In Mumbai?

>> Yes.

>> ROTELLA: After his arrest, Headley gave up his friend, testifying that Rana was his accomplice.

At trial, Rana denied it, but he was convicted and sent to prison.

>> So many innocent people, they are killed in this Mumbai attack.

David Headley is insane.

That's it.

I can say only this thing.

No person with a brain can do these things.

He's insane.

>> ROTELLA: David Coleman Headley is not his given name.

He was born Daood Gilani in Washington, D.C., in 1960.

His father was a well-known Pakistani broadcaster, his mother, Serrill Headley, a daughter of Philadelphia high society.

The family moved to Pakistan early in the boy's life.

One neighbor was this man, Chand Bhai, who says he's known Gilani since that time.

>> ROTELLA: But soon the parents divorced, the clash of American and Pakistani culture at the heart of the break-up.

His father remarried, his mother returned to America, forced to leave her son.

Daood grew up in private military schools, where duty to flag and country was instilled.

>> Every morning at the Habib Public School, hundreds of boys line up to pay tribute to the flag.

>> ROTELLA: He says he was a student here when Pakistan suffered a humiliating defeat in a war with India.

Stray bombs fell on the school, killing two people.

>> ROTELLA: At home, Daood didn't get along with his stepmother, and at age 17 he sought the help of his true mother, Serrill Headley.

She brought him back to the U.S., to Philadelphia, where she owned this bar, the Khyber Pass.

Here, Daood met American culture head-on.

Thanks in part to his mother.

>> Hello, everybody, and welcome to the Khyber Pass Pub at 2nd and Chestnut Streets.

We are live tonight.

>> ROTELLA: She was a fixture in the Philadelphia social scene, a local character known for her bar... >> The Khyber Pass Pub is ten years old... >> ROTELLA: ...and her colorful past.

>> ...the current and what you might call a very special owner.

>> She is a Delaware Valley native, but ten years ago, her story read like a Mideastern spy novel.

>> ROTELLA: A past she happily advertised-- life in Pakistan, false charges of espionage, her life under threat, and escape through the bar's namesake.

When her 17-year-old son arrived from Pakistan, it only added to the bar's mystique.

There, they called him "The Prince," but a prince who had a dark side.

Daood Gilani, age 23, handsome, self-assured, and within two years of these images being taken, a heroin addict and a budding drug smuggler.

It's hard to pinpoint exactly when Gilani stepped onto the path that eventually led to the Mumbai attacks.

Headley has now moved overseas to Philadelphia, but he's visiting Pakistan and apparently already has this drug habit.

So he goes up to the tribal areas with Rana, his best friend, because Rana has a military ID, unbeknownst to Rana that he is going up there to get drugs.

So he uses him essentially as cover, and they make it back, but a couple days after he returns, he gets arrested in a hotel with a woman, and there's some kind of incident that causes a commotion and draws the attention of the authorities, and Headley gets arrested for drug possession.

But somehow, Gilani got away with it.

Then in 1988, he got caught in transit by U.S. drug agents.

Two kilos of Pakistani heroin tucked into the false bottom of his suitcase.

On the spot, he agreed to cooperate with the DEA, the Drug Enforcement Administration.

One partner got ten years.

The other got eight.

Gilani only got four.

>> He just turns around immediately and betrays everybody when it's convenient for him.

Basically, it's survival for himself.

>> ROTELLA: Gilani did his time.

He moved to New York.

But three years later, he was arrested again for drug smuggling.

It's a turning point in his life, and he writes about it in a document I was given.

This is the first time it's been made public-- a draft memoir, his life in his own words.

During his second stint in prison, Giliani writes that he "rediscovered the seeds of Islam sown in him by his father."

He says he wanted to leave behind his unrighteous ways.

And to make amends, he would try to work with the Drug Enforcement Administration as an informant.

More importantly, he wanted to get out of jail.

>> ROTELLA: Pleasure.

>> Likewise.

>> ROTELLA: He hired a lawyer to negotiate terms.

>> I remember him being highly intelligent, understood what his situation was, had a clear idea of what he wanted to do.

>> ROTELLA: Which was?

>> What he wanted was to cooperate with the government, which he had done previously.

>> ROTELLA: He cooperated.

His job was to set up his sources in Pakistan.

The DEA sent him there one time.

>> That happens because there was a lot of suspicion that maybe he was simply trying to set people up.

But if he had the ability to physically travel all the way to Pakistan and show his face, that that would allay concerns.

>> ROTELLA: But he says he went more than once, without their knowledge.

On one trip, Gilani made contact with Lashkar e Taiba, a militant Islamic group affiliated with this mosque in Lahore.

Its mission spoke to Gilani, to his Pakistani identity and his hatred of India.

Lashkar was dedicated to the fight with India over the disputed territory of Kashmir.

Lashkar is known for its technical and military sophistication and its ties to Pakistan's intelligence agency, the ISI.

Gilani writes that on another trip, he met Lashkar's leader, Hafiz Saeed, and was moved by his militant call to action.

One second spent in jihad, he said, is superior to 100 years spent in worship and prayer.

Gilani was hooked.

>> He decides to actually join Lashkar e Taiba while he's still a source for the DEA.

>> ROTELLA: When he wasn't in Pakistan, Daood Gilani lived here, on Manhattan's Upper West Side, near a video store he owned.

Gilani's life was in flux.

He was married to a woman in Pakistan and seeing women in America.

But he'd given up drugs and was immersing himself in radical Islam.

And then...

The day that changed everything and everyone, including Daood Gilani.

The next day, his DEA handlers called him.

They needed more than just a drug informant.

Now they wanted to know about terrorists.

He cooperated, gathering intelligence on extremists in New York and calling sources in Pakistan.

But he revealed other views to an ex-girlfriend soon after 9/11.

She then told her friend, Terry O'Donnell.

>> ROTELLA: You guys were sitting in a bar, right, you and her and... >> We were standing.

>> ROTELLA: You were standing, all right, you, her and somebody else?

>> Another waiter, but he was kind of hitting on a girl two stools down.

Sportscenter was on here, and Channel 7 news was on here, and it was about 11:30 at night and the news was on.

She said, "Well, you know, my boyfriend said America got "what it deserved, you know?

I mean, we're not innocent in this."

And I was like, "Wow, that's a pretty insensitive thing to say."

And then she went on and said, "he was happy to see it happen," and "he got off on watching the news over and over again."

I was conflicted whether I should say something to the cops.

I don't know, maybe this guy's just all talk, he's just saying this, he's an [bleep].

He's entitled to his opinion.

Does this mean I should call the authorities on him, like the police, or...?

And then, you know, you look downtown and you're like, "I don't know, maybe this guy is for real."

>> ROTELLA: Tipped off by O'Donnell, the FBI questioned Gilani about his statements.

He had also been overheard boasting of going back to Pakistan to fight the Jihad.

With DEA agents in the room, he denied it all.

>> In October of 2001, he was confronted by FBI agents.

He said, "Oh, well, you think I'm an extremist?

"You think I'm interested in jihad?

"Don't forget I'm working for the U.S. government.

"You've got it all wrong.

"I'm one of the good guys.

I'm working for the DEA."

And he uses that information to cover allegations of his extremism.

>> ROTELLA: He talked his way out of it.

Soon after, at a hastily called hearing, a U.S. prosecutor asked a judge to end Gilani's probation early, a highly unusual move.

>> It's the only occasion I can recall it ever happening.

>> ROTELLA: Howard Leader was there for Gilani.

He says U.S. officials seemed to want to rush his client to Pakistan as an operative in the new war on terror.

>> I think that he was going to go back to Pakistan with a view towards meeting with or gathering whatever information he could that might be useful to the U.S. government regarding certain extremist elements there.

>> ROTELLA: Intelligence officials say they knew about Gilani's involvement with Lashkar, which would soon be designated by the U.S. as a terrorist group.

But it's not clear if they understood his growing commitment.

In fact, he was becoming more entangled with Lashkar by the day.

By early 2002, he had made his way to its camps in the northwest mountains near Kashmir.

Soon, he was taking military training courses offered by Lashkar.

>> Gilani would have started out with a religious indoctrination.

And several months later, he would have gone on and done the general training, the three weeks' light weapons.

And sometime later, he went on and he did the specialized training, and that's hand-to-hand combat, guerrilla warfare training, irregular warfare training.

>> ROTELLA: The memoir recounts that during his training, Gilani was taken to Kashmir to see how Lashkar fighters infiltrated into India.

>> Lashkar e Taiba is opposed to suicide bombing.

They believe it's a sin for you to kill yourself.

And so what these people are going to do is they're going to fight to the death.

They're gonna die by the enemies' hand rather than their own.

Lashkar began launching raids by a small number of people in Kashmir.

And sometimes these were hit-and-run attacks, like you would normally get in battle, but quite often, what they did is they would hit and then they would stay.

They didn't run.

They hunkered down, and it was sort of this stronghold option, and the idea was that they would fight for hours upon hours.

Sometimes 20, 25, 30 hours these battles would go on.

>> ROTELLA: The DEA says it officially deactivated Gilani as an informant in early 2002.

But there are conflicting versions about when he actually stopped cooperating with them.

Investigators and intelligence officials say the information gleaned from him was of little value.

Hopes of using him to spy on terrorists soon fizzled out.

If Gilani's terrorist connections were not causing official alarm, they worried his friends and relatives, including his mother.

Serrill Headley moved to this house in the small town of Oxford, Pennsylvania, in the late 1990s.

On many days, she would come here for coffee to talk with Phyllis Keith, who owns the Morning Glories cafe with her husband, Michael.

>> She came in regularly, I'd say maybe two, sometimes three times a day.

>> ROTELLA: And at some point, some of the stuff she said started to pique your interest.

Can you tell me about that?

>> What I remember is it was later in the day, and there weren't any other customers in the shop.

And she sat down and said, "I think my son might be involved with training camps in Pakistan."

Just pretty much straight out said that.

>> The impression I got from her is that he was in and out of the country pretty regularly.

And at times, she wouldn't know where he was.

>> At that point in time, they were saying, you know, "If you hear something, you see something that makes you suspicious..." And one night, I went home from work, got out the phone book, looked up the FBI, and gave them a call.

>> ROTELLA: You think the conversation lasted how long?

>> I don't know.

Five minutes?

>> ROTELLA: Did you hear from them again?

>> No.

>> ROTELLA: As Daood Gilani honed his skills in Lashkar e Taiba's training camps, he was becoming a Pakistani warrior.

And after three years, he wanted to fight in Kashmir.

But one commander in Lashkar had other plans for him.

His name: Sajid Mir.

This is the only known picture of Mir.

He was in his 30s at the time, a technically sophisticated new-generation leader who wanted to emulate Al Qaeda's international jihad.

He spoke fluent English and led Lashkar's recruitment of non-Pakistani operatives.

His terror plots were known, at least to some in Western intelligence.

For years, Jean-Louis Bruguiere was a top anti-terror judge in France.

He began investigating Lashkar in the early 2000s.

He says he warned high-ranking U.S. security officials about the threat it posed.

>> ROTELLA: Who is Sajid Mir?

>> ROTELLA: Bruguiere's investigation led to Sajid Mir's conviction in a French court, in absentia.

Mir was in Pakistan at the time.

To this day, investigators say Mir is untouchable, protected by the most powerful branch of the Pakistani military: the ISI.

>> Sajid Mir, is he really an ISI person who is within Lashkar e Taiba?

Or is he a Lashkar e Taiba person who was trained by the military in the background?

It doesn't matter.

It doesn't matter because in a sense, Lashkar e Taiba was a proxy of the Inter-Service Intelligence directorate and very much under their control.

>> ROTELLA: By 2005, Mir had plans for Daood Gilani.

He was now a Lashkar operative in training, being readied for an undercover mission.

But he wasn't an international spy.

Not yet.

>> Lashkar e Taiba is a terrorist organization, so they train people to kill.

They don't do logistics very well, and Headley needed real training in espionage.

>> ROTELLA: The training would come from the ISI, a man we know of as Major Iqbal.

It's probably an alias.

INTERPOL has him listed as wanted, but they don't even have his picture.

Iqbal took the new American recruit and oversaw his training.

So the ISI could maintain deniability, Gilani says he was always to report to Iqbal and Mir separately.

>> Iqbal delegates a non-commissioned officer to give Headley additional training in terms of espionage.

>> ROTELLA: So now you've got a guy who's gone through all this Lashkar training, more than most Lashkar militants do, and now he's done this additional espionage training with the ISI.

How does he compare to other jihadis of the many you've looked at?

>> Gilani is, you know, a gold mine for both an intelligence service and a militant organization that is looking to gather information.

>> ROTELLA: He was now Sajid Mir's agent.

On orders, Gilani returned to the United States in August 2005.

His first stop was in New York.

His Canadian-born second wife was running his video store.

The couple was having trouble.

This used to be Flik's Video, which was the video store that Headley owned for a number of years.

He comes back here from Pakistan and he meets with his wife at this store, and they have an argument about money, and the allegation that the wife made at the time was that he got angry and he hit her.

Apparently she said he backhanded her with his cell phone in his hand.

She had Gilani arrested for assault, but the case was ultimately dropped.

She also reported him to the FBI.

She met with agents three times and told them in detail about Lashkar, about the training camps, even that Gilani had bought night vision goggles.

You know, this incident that happened here with the combination of the domestic assault allegation and the tip to the FBI represented a golden opportunity to find out who he was.

This was a serious moment, like a hinge moment in his trajectory into terrorism.

The FBI called Gilani's former handler at the DEA.

By now, Gilani had stopped working for them, but law enforcement officials say that his past as an informant caused the FBI to drop the inquiry.

It was as if Gilani could get away with anything.

His next stop, here at the City Hall in Philadelphia, was to follow through on Sajid Mir's orders.

I was looking for a record of a name change.

Gilani, G-I-L-A-N-I.

>> He comes back the U.S. and he changes his name to David Coleman Headley so that he can travel more easily, more covertly.

>> ROTELLA: "David" was English for Daood.

Coleman was his grandfather's name and Headley was his mother's maiden name.

It was a simple bureaucratic act.

But U.S. intelligence officials say changing his name would make it just that much more difficult to keep track of him, even though he was working with Sajid Mir, a terrorist known to Western intelligence, and Lashkar, a group in the electronic sights of U.S. and British spy agencies.

What is interesting is there is already a net, right, over people like Sajid Mir, and Headley is communicating and meeting with those people, yet his contacts with these people who were now under this spotlight weren't detected.

>> I can't speak to why he wasn't detected, as you say, when he was communicating with other people that were of concern and that were on our radar.

Perhaps there would be an awareness that there was an individual who was taking on this role or who had this alias or something along those lines, but not be clear that it was David Headley, an American citizen.

>> ROTELLA: The name change, the training, all the preparation was, by mid-2006, coming together for what was David Coleman Headley's ultimate mission: Mumbai.

Over the course of 20 months, Headley traveled in and out of India at least eight times, staying weeks or months at a stretch.

Each visit, he advanced the attack plan.

>> Investigative sources say Headley surveyed all the 26/11 targets.

>> The game is over for Lashkar operative David Coleman Headley.

>> ROTELLA: I had come here in 2011 to retrace his steps.

>> As we know, David Headley was an undercover agent working for... >> ROTELLA: Now I was back looking for new evidence: the digital clues he might have left behind.

In the physical world, Headley had the edge.

He looked nothing like a Pakistani and spoke fluent Hindi.

He insinuated himself into the city: walking the streets, living like a local.

Deven Bharti is a top police official who investigated the Mumbai attacks.

>> After this place, they took a left turn.

>> ROTELLA: He questioned the people who knew Headley, like his secretary.

Headley hired her for the Mumbai office of the immigration company owned by his friend Rana.

The business was here, but it was just a front.

>> ROTELLA: But if someone had been looking, he was leaving clues in the most visible places.

The Taj Mahal Palace Hotel, an opulent icon of India, built in 1903.

From the beginning, Sajid Mir knew this would be ground zero of his attack plan.

Headley stayed here on his first reconnaissance trip in September 2006.

Months into the mission, he even checked in with a new wife on their honeymoon, paying with credit cards.

He bought meals in hotel restaurants and a designer bag here, purchases that would later digitally link Headley to Lashkar and terrorism.

We know specifically because of the credit card charges that he went to the Mont Blanc store and he went and had breakfast in the Sea Lounge with that wonderful view of the water, and he's videotaping and he's just assembling all this information that he's gonna use this place, enjoy it, and then he's going to be the engineer of its destruction.

As Headley cased the Taj Hotel and other potential targets, he was leaving behind a digital trail of clues-- if a spy agency had been looking.

Headley kept at least five email addresses while in Mumbai, used to contact his Lashkar and ISI bosses.

And he sent long emails to his high school friend, Tahawwur Rana, who wired Headley money to help him create the business front.

And at one point, he traded a flurry of 18 emails with Sajid Mir and Major Iqbal about one potential target: India's Hindu nationalist party, Shiv Sena, a sworn enemy of Lashkar.

Come in.

Raja Rege worked with Shiv Sena.

Headley had befriended him and enticed him with fake business opportunities.

In his memoir, Headley says he got complete access to the group's headquarters, filming inside.

And what happened next?

>> ROTELLA: And then he starts exchanging emails with you.

>> ROTELLA: Did you see him again?

>> No.

>> ROTELLA: So all this is going on by email?

>> Emails, yeah.

>> ROTELLA: You know he was working for both Lashkar e Taiba, the terrorist group, and the ISI, the Pakistani Intelligence Agency.

When he communicates with you, he writes to them very excitedly.

He saw you as a target to potentially do espionage and even terrorism.

I mean, that must've been a powerful thing to realize.

One idea was to offer the leader of the party a trip to America where, Headley wrote, "We can take care of them"-- code for an assassination plot.

>> ROTELLA: Headley and his other handler, Major Iqbal, traded emails with terms that could have raised flags for intelligence analysts.

He even addressed his handler by name and rank.

He was communicating with plotters and groups known to international spy agencies.

But no one in the intelligence world seemed to have noticed.

When he returned from his reconnaissance trips, Headley found his status in Lashkar was growing.

He was the American operative who did things nobody else could.

In his personal life, Headley adopted the ways of Lashkar.

Already married to two women, Headley decided to marry again, to a Moroccan named Faiza.

>> ROTELLA: She was fiercely independent.

A med student, she wore Western clothes and partied with Headley's old friend Chand Bhai.

He said Headley wanted a more traditional Muslim wife.

So he wanted her to wear... >> ROTELLA: She changed for him, but Faiza felt she was being treated as a mistress.

She was left alone for months while he was in Mumbai.

The marriage fell apart, and her anger led to what happened next.

She goes to the U.S. embassy in Islamabad and warns them about her husband's extremist activities just as the Mumbai plot is really gathering momentum, the reconnaissance and the preparation.

>> That's right.

They must have had a disagreement.

She, you know, she's short fused, she goes to denounce him and mentions that he was trained by Lashkar e Taiba, he's really a terrorist, and nothing happens.

>> ROTELLA: She met with embassy officials three times between December 2007 and April 2008.

She told them her husband had ties to the ISI and hated India.

And perhaps most surprising, Faiza told U.S. embassy officials about their honeymoon at the Taj the year before.

In combination with her other charges, that could have led investigators directly to Headley's connections to Lashkar and the ISI.

But intelligence officials at the embassy declined to look at the case.

Embassy security officials filed it away as low priority.

An opportunity was lost.

During the same period, the U.S. began picking up chatter-- disconnected bits of intelligence-- but none associated with Headley.

>> One of the difficulties with that is Lashkar is always plotting.

In 2008, as previous years, we knew that they were engaging in plotting, and that information had been shared with the Indians.

But it wasn't clear where they were on a number of these attacks.

>> ROTELLA: As 2008 progressed, the U.S. kept hearing more chatter.

Senior U.S. officials have told us that the NSA was collecting intelligence about a Lashkar threat to Mumbai.

Later, the CIA intercepted a communication indicating an attack might come by sea.

Counterterror agents at the U.S. embassy in New Delhi relayed what they knew to their Indian counterparts.

As a result, Indian police issued a warning in September that listed the Taj Hotel at the top of a possible list of targets.

Security at the hotel was temporarily beefed up, but little more was done.

>> We got warnings that there was likely to be an attack in Mumbai.

The Taj Hotel was very specific, but it's like any other thing: you put an alert, people will wait for 15 days of alert or 30 days of alert, and then nothing happens.

>> ROTELLA: Early in Headley's reconnaissance, his handlers had a limited goal.

But his reports seem to embolden Lashkar.

>> What starts out as this one- to two-person hit-and-run attack against the Taj Mahal Hotel becomes this ten-person multiple target attack of the kind that Lashkar e Taiba has never launched before.

>> ROTELLA: By early summer 2008, a date was set for the attack.

And another important decision: the attackers would hijack an Indian fishing boat and sail to Mumbai.

David Headley comes here and one of the crucial roles he plays is in setting up a maritime attack, which is the hardest kind of terrorist attack.

Having the gunmen arrive by sea is harder than having them arrive in any other way.

He took boat tours from here.

He hired a fisherman to take him around.

And what he was looking for was the best approach and the best landing site.

Headley scoured the city, searching by car and boat.

And then he found the perfect spot: a fishermen's slum.

It's a pocket of poverty amid wealth, its beach used as a public sewer.

Here, he saw a strategic landing site.

A main thoroughfare runs nearby.

Headley brought a GPS unit to map it out.

He comes here.

He plots the GPS coordinates for this landing spot.

This route from Karachi to Mumbai had been plotted out by GPS.

This is the route, and this is where we are right here.

With a video camera, Headley traced the routes for the attack, the Taj Mahal Hotel about eight blocks east.

Two blocks closer was the Leopold Café, a favorite for Western tourists.

Just around the sea wall was the Oberoi Hotel.

And then, a quick taxi ride away, locals would be targeted at the busiest rail station in Mumbai.

Headley soon left for Pakistan with the intelligence he'd gathered.

He returned on July 1, 2008.

Airport security cameras captured this picture of him upon his arrival in Mumbai for his last reconnaissance.

Major Iqbal and Sajid Mir had given him final instructions.

Now he returned to the Taj to scout the attack's main target.

He came here to the lobby, using the camera on his mobile phone to film.

These images have never been seen before.

He writes in his memoir that an employee told him to put down the camera.

Video-taking was not allowed.

There was one more target to pinpoint: Chabad House, a synagogue and hostel run by an American rabbi and his wife.

Major Iqbal chose the target.

His objective: the global jihad against Jews and Americans.

In his memoir, Headley writes that he looked for security cameras.

He videotaped, and he took these pictures.

He writes that the building seemed inconsequential.

He took the risk of calling Major Iqbal.

"It's not as valuable as you think," he says he told him.

Major Iqbal replied, "No, Mr. Headley, a lot of real estate "experts have told us that that particular property is going to appreciate in value within a few months."

"He was laughing," writes Headley.

He sent more email, used his credit card-- all potential electronic evidence of Headley's mission that, after two years, was over.

In August, he returned to Pakistan, here to the Gilani family compound in Lahore.

In his memoir, he explains that Lashkar was moving forward with the Mumbai attack plan.

The attackers, the boys, would carry GPS devices.

Attack leader Sajid Mir took Headley's reconnaissance video and GPS data and gave them to a man who helped train the attackers.

His name: Zarrar Shah.

He was Lashkar's communications chief.

His team of young computer experts were 21st-century jihadis.

I learned a lot about Shah from the interrogation of his assistant, who was captured and questioned by Indian authorities.

In his interrogation, he said Shah was in charge of setting up a way to talk to the attackers once in Mumbai: a computer-calling system, known as Voice over Internet Protocol, or VOIP.

In 2008 in Pakistan, it was a cutting edge technology that they thought was harder to trace.

They were wrong.

What Shah never knew was that Western spies had gained access to his online communications months before the attack on Mumbai.

It was the British equivalent of the NSA: General Communications Headquarters, or GCHQ.

>> So this is 2011... >> ROTELLA: We found the evidence in Edward Snowden's trove of documents.

Jeff Larson reports on technology and intelligence for ProPublica.

He was the first to analyze the Snowden documents related to Mumbai and Zarrar Shah.

>> It's a long, arduous process to search through this material because it's so gigantic.

There's tens of thousands of documents.

So I started looking for information about what happened in Mumbai and realized that they know about Zarrar Shah.

>> ROTELLA: It was a startling revelation.

Western intelligence had technology in place to monitor Shah before the attack on Mumbai.

>> It looks like he became a target of interest.

They had been following him around the Internet.

You know, it looks like they found sort of an operations guy.

>> ROTELLA: Right.

>> Once they have enough information via just tapping the Internet, all of a sudden, that person becomes a target.

>> ROTELLA: Shah's emails, searches, and other online activities were being captured.

But it is unclear if anyone was actually analyzing the stream of intelligence.

So we have now this time frame, September.

What we don't know is how much is collected, how often it's reviewed, if it's collected, is it analyzed, right?

>> Right, exactly.

I mean, we don't know.

In other stories, we've definitely seen that when they get, like, an intelligence tip, they're on this person immediately.

There are phases that this, you know, sort of intelligence collection is going through.

They might be collecting it in case, you know, they get another tip and you get more and more interesting.

>> ROTELLA: From Shah's assistant, we know that in the days before the Mumbai attack, Shah set up the attack control room.

It was in the Pakistani port city of Karachi, on the second story of a building whose ground floor was a fishing supply business.

Intelligence sources gave me this layout of the control room.

Two televisions on one wall, four laptops connected to the Internet.

The target in Mumbai was 500 miles away by sea.

Lashkar leaders gathered in the control room as the attackers made their way to Mumbai on the hijacked Indian fishing boat.

Sajid Mir was in charge.

Zarrar Shah was near him, working on a Lenovo laptop connected to the Internet.

At hand would be David Coleman Headley's surveillance footage and GPS coordinates that he had collected.

All of it was leading to November 26, 2008.

That evening, Headley was at home in the city of Lahore.

He had been waiting for a message from Sajid Mir in the Karachi control room when he received a text on his phone.

Mir wrote, "Turn on your television."

>> The gunmen struck at hotels, stations and restaurants across Mumbai.

Hundreds have been injured and there are reports of Western hostages taken.

>> This is possibly the most well-coordinated attack... >> The layout of the hotel... >> The attackers were captured on closed-circuit television.

>> Officials describe this as a professional and highly coordinated... >> There were explosives, some weapons.

They were able to lock down... >> Teams of gunmen fanned out across the city.

>> There are people trapped in that building, there are people still in this Taj Hotel.

>> This is a huge, massive fire that is on top of the Taj.

>> The Taj Hotel.

>> The Oberoi Hotel.

>> Cafe Leopold.

>> And a train station.

>> India has seen terrorism before, but nothing that would have required this level of planning and coordination.

>> At least 150 people have been killed.

>> One captured gunman is Pakistani.

>> 24 hours after these multi-pronged coordinated attacks began, this crisis is still going on.

>> A Jewish community center was attacked, the Israeli family taken hostage.

>> ROTELLA: Chabad House is not easy to find, but Headley's GPS had guided two of the attackers to its location.

When I first came here in 2011, it was still pockmarked from the bullets and RPGs of the attack.

The attackers had taken hostages.

This began a three-day siege.

>> They had a lot of ammunition, and they kept on going for a while.

>> ROTELLA: Moshe Holtzberg's brother, Rabbi Gavriel Holtzberg, and his wife, Rivka, were among the first to die.

>> This is where they found my brother and his wife, lying over here.

>> ROTELLA: Indian authorities were able to intercept the attackers' cell phone calls.

This is when they overheard the conversation with one of the hostages.

>> It was Sajid Mir, directing the siege from Kirachi.

>> He wanted a bargain for his wounded attacker, he reassured Norma.

>> ROTELLA: A proposed prisoner swap didn't happen.

Then another call from Sajid Mir.

This time, he's speaking in Urdu.

(gunshots) >> ROTELLA: It was the beginning of the end of the Chabad House attack.

Indian commandos finally arrived.

Their assault on the Lashkar attackers was broadcast on television.

Sajid Mir, in Karachi, watched live.

>> ROTELLA: But there was something else.

The documents leaked by Edward Snowden say that just hours after the Mumbai attack began, British spies realized something incredible.

They could spy on the attack control room.

>> As the attack is happening, GCHQ noticed that they can start collecting, or listening in, on the command center.

>> ROTELLA: It included access to Shah's electronic communications.

>> They can essentially watch over Zarrar Shah's shoulder as the attack happens.

You have, in theory, access to all of the websites that they visit, you know, things that they're looking at on Google maps, the whole bore of everything they're doing at any given time.

>> ROTELLA: Shah's computer was serving as a direct line for Sajid Mir to the attackers at Chabad House, his voice crystal clear as he talked with Ukasha, one of the attackers.

(gunshots) (rapid gunfire) >> ROTELLA: The gunshots signal the final moments of the Chabad House siege.

>> Ukasha?

Ukasha?

>> ROTELLA: Two terrorists were dead.

Six hostages had been murdered.

(phone beeping) The Mumbai attacks ended the next day.

166 people had been killed indiscriminately-- dozens of nationalities, a spectrum of religions, including Muslim.

Despite close ties between British and American spy agencies, U.S. officials say that only after the killing was underway were they told by GCHQ about the access to Shah.

The British also informed Indian intelligence, which it turned out had been monitoring Shah before the siege as well.

What's remarkable to me is, you know, it would be very easy to just jump to the conclusion, like, "My god, how could this have not led to the prevention of the attack?"

This is a huge piece of the puzzle.

It was an historic near-miss, analysis and intelligence sharing that didn't happen.

One NSA document says the British pre-attack data on Shah later helped analysts piece together Lashkar's complete operations plan.

GCHQ disputes the idea that it had information that could have stopped Lashkar's plot, and says it would have shared any such intelligence with India.

The Snowden documents show that GCHQ considered the case to be a notable success.

>> The odd part about this is in write-ups after the fact, they do say this was a successful operation, that things worked the way they're supposed to.

But the facts on the ground is Mumbai still happened.

>> ROTELLA: India and the world were reeling.

Lashkar e Taiba was no longer in the shadows.

The detailed intelligence from the control room gave Western agencies evidence to force Pakistan to crack down on the group.

Pakistan denied any role in the Mumbai attacks.

Within days, Zarrar Shah and other top Lashkar chiefs were arrested.

But not David Coleman Headley.

He was at home in Lahore, still unnoticed by international intelligence, even though he did little to hide himself in cyberspace.

During the attack, his wife wrote an email of praise about his work in Mumbai in barely coded words.

"Congrats on your graduation," she wrote.

He received other emails with grisly wire photos of the attacks, and then he forwarded them to another email account he owned.

In an online chat room, Headley even boasted he had inside details of the attacks.

Just two weeks after the siege, Headley traveled back to the United States, leaving a path of records and border crossings.

The NSA had now intensified its monitoring of Lashkar and the ISI.

U.S. intelligence officials say they ended up catching some of Headley's emails in the net.

But they admit they didn't connect the dots.

There's information gathered in terms of his emails and his calls and his contacts.

One wonders why he didn't get detected earlier.

>> It's partly about volume.

You might be able to access all that stuff, but you can't collect it all, you can't store it all for very long, you can't analyze it, so without some little indications of where the needles are, you're looking at an aircraft hangar full of haystacks.

It's about collecting, analyzing and warehousing unimaginable amounts of data.

The world sends three million emails a second.

>> Your communications, my communications, online gaming, massive pornography, Netflix streaming of films, financial markets talking to each other.

And the intelligence task, and it's a very big one, is to try and find that very small amount of information which has value.

>> We have this giant program predicated on the idea that we're gonna suck up all the communications in the world and feed them into some computer, and that's how we're going to discover whether terrorist attacks are going to occur.

The question is, does it work like that?

They did not just look at all of the metadata for every cell phone they're intercepting and then it somehow flashed up on the screen and told them David Coleman Headley is a bad guy.

>> You need someone to tell you or something to tip you off, to say that this guy, David Coleman Headley, is interesting.

>> ROTELLA: All along, there had been plenty of signs that Headley was potentially dangerous, tips stretching back seven years.

And just days after the Mumbai attack, Headley by then back with his family in Chicago, investigators were looking into another tip.

It came to the FBI's Philadelphia office from a woman who had known Headley's mother.

>> She sees these attacks on Mumbai and she has a friend whose son may have been over in Pakistan fighting, and so she contacts us.

And so our Philadelphia division goes out and interviews Headley's cousin.

We talk to him, and he states that yes, you know, he is in Pakistan, but knows of no information in terms of Headley being involved in any plot.

What we know now is Headley was actually in Chicago during that time frame.

>> ROTELLA: So what the cousin said to you guys wasn't true?

>> That's correct.

>> ROTELLA: It was another lost opportunity.

If the Philadelphia tip had been pursued, incriminating communications could have been detected: A call from his cousin warning him the FBI had visited.

Emails Headley wrote to jihadi contacts in Pakistan saying he was worried the FBI was interested in him.

Other communications asking if Sajid Mir had been arrested after the Mumbai attacks.

And this: an email about something he and Mir had been discussing for months.

More code words, but also terms any investigator would question.

It was a new plot.

Just weeks after the Mumbai attacks, Headley was preparing to go to Denmark to do reconnaissance.

The target: Jyllands-Posten, the Danish newspaper that had published cartoons of the prophet Mohammed.

Headley's mission was to take revenge on those who had published the cartoons.

As he'd done in Mumbai, Headley moved with stealth.

His cover once again, a U.S. businessman.

He stayed at this hotel as he scouted targets.

>> He was here until the 22nd of January.

>> ROTELLA: From the 17th to the 22nd?

>> Uh, yeah.

>> When Headley came by to visit Jyllands-Posten, I guess I had been working there for about six months.

We weren't really used to businessmen walking through the door.

>> ROTELLA: Headley surveyed his target, taking this photo of the Jyllands-Posten building.

And on January 19th, Headley sent a progress update to Tahawwur Rana, the same man who provided a business cover for him in Mumbai.

He says that surveillance is going well and that he is going to find out about placing an ad in Jyllands-Posten.

And he doesn't hesitate to contact the newspaper's ad department by email.

The man who helped plot the Mumbai attacks had walked into one of the top targets in the world.

But no one was looking for David Headley, so no one noticed.

After ten days on the ground, Headley headed to Pakistan.

>> In the wake of the Mumbai attacks, Lashkar begins to come under increasing pressure, and ultimately they tell Headley to put the attack in Denmark on hold.

Headley being the entrepreneurial guy that he is wants to continue to move this forward, and he finds new support in the form of Ilyas Kashmiri, who is Al Qaeda's director of operations in Pakistan.

>> ROTELLA: He told Headley that those above him supported the Denmark plot.

Headley was now working for Al Qaeda.

It changed the nature of the attack.

>> The plan was taking the editors hostage and beheading them and throwing their heads off the roof.

This is not just a small attack against the paper, a vengeance attack.

This is going to be a shock and awe attack in being so graphic, in being so morally incomprehensible.

>> ROTELLA: To get the job done, Headley would need logistical support.

Kashmiri gave him money and the names of two contacts in the UK.

Headley then flew back to Chicago.

Once there, he called Kashmiri's men.

But that act-- placing a phone call-- was Headley's biggest mistake.

Kashmiri's men were under surveillance by British counterterror officers.

The British alerted the Americans.

The response to this tip would be different.

>> It was a telephone number and a first name only.

An individual named David in contact with two individuals that were of interest of our foreign liaison partner.

>> ROTELLA: The tip mentioned Al Qaeda and came from a trusted ally.

Still, it was just one of many leads coming in.

It was handed off to the newest FBI agent on a counterterrorism squad.

>> I arrived on the squad on July 17th, and on July 22nd, I received from my supervisor a lead.

>> ROTELLA: This is your first lead too, right?

>> Yes.

A partner and I drove out to the actual location to confirm that that telephone was the number that had been in question.

It was a public payphone.

This caller, David, last name unknown, had used this payphone more than once.

>> ROTELLA: They were unaware how close they were to their suspect.

The payphone was just blocks away from Headley's apartment.

>> We began to realize that he was going to travel outside of the United States.

>> ROTELLA: And then you start working with customs and border protection?

>> Correct.

They're scrubbing their flight data, their manifests, they're looking for Davids that were traveling during that time frame.

>> ROTELLA: At what point do they come up with the name?

>> So on July 25, CBP confirmed that there was a David Headley specifically on that flight that was identified.

I notified our foreign partners that he was en route to the location that they were monitoring.

>> ROTELLA: He was on his way here: Derby, a city in Northern England.

It's where Kashmiri said Headley could find support for the Denmark plot.

>> It has served as a hub for terrorist activity.

Derby has a very prevalent second, third generation Pakistani community, but the connections with Pakistan are very strong.

Through their lineage, they are actually entitled to Pakistani citizenship.

Therefore, the ease of travel to Pakistan becomes quite straightforward.

You have individuals who are recruited to go fight in insurgencies in Afghanistan and Pakistan, and then bigger causes, bigger groups, like Al Qaeda.

>> ROTELLA: He met with the contacts.

We know them as Simon and Bash.

Headley was looking for guns, money, and volunteers.

It didn't go well.

>> Simon and Bash were worried that they were being monitored, that they were potentially being watched, so they felt more comfortable talking to Headley at a bus stop.

>> ROTELLA: He comes in thinking that Ilyas Kashmiri, this incredibly powerful terrorist, has got two operatives waiting to meet him, to give him $10,000 in guns and either participate in the attack or provide men who will, and there's a total disconnect.

And he's totally crestfallen when they immediately make clear to him they don't want to participate in this attack.

>> The UK was monitoring Headley's activities once he was in Derby and providing that information to the U.S. and also to the authorities in Denmark.

>> This is the road going to the palace.

>> ROTELLA: Dejected, but still determined to pull off the attack, Headley went to Denmark on his own.

But this time, he wasn't alone.

Danish intelligence agents followed him as he cased Copenhagen right up to the front entrance of his target, Jyllands-Posten.

But he had a problem.

>> He becomes very confused because the paper has moved.

You know, if you're just focusing in on the paper, if you know the location, then he comes back and then it seemingly had moved, there is another level of complexity.

>> ROTELLA: He headed back to Chicago with diminished ambitions.

If he couldn't take down the paper, he would take out the cartoonist.

It had only been a week since the FBI had identified him, and it was only as he flew home that agents learned of Headley's connection to a key Al Qaeda figure.

>> It's when we make the connection with Headley and Ilyas Kashmiri that is, you know, "We have something here.

We have someone that's connected to core Al Qaeda."

The concern is, is there a homeland plot here in the United States?

>> Now this investigation had garnered the attention all the way up to the Attorney General.

>> ROTELLA: They opened up on Headley, monitoring his phones, his home, his car.

>> We received the authority to go up on Headley's communications real-time.

Anybody that came in contact with Headley became a target of ours.

>> And that's when it kind of grew into a larger picture of who David Headley really was.

>> ROTELLA: The FBI was combing through large amounts of data, and at their request, the NSA opened its vast archives.

Years of domestic and international surveillance, phone call records and electronic communications like emails were searchable.

>> We had people working literally around the clock, here in Chicago and at FBI headquarters, to scrub through this information.

>> ROTELLA: Any receipt, any email was a tiny glimpse inside the mind of a terrorist.

Soon, they connected him to Lashkar and the ISI.

But his role in the Mumbai attack remained hidden within the cascade of data.

(gunshots) (explosion) >> There was an email from his wife to Headley that said "Congratulations on your graduation."

>> We did not understand what necessarily that email by itself meant.

Just a few days prior to the arrest, we started putting the pieces together initially of Headley's travel into Mumbai with a credit card receipt at a store at the Taj Hotel.

>> ROTELLA: The clues added to a larger case the FBI had built with traditional investigative work-- the wiretaps, the stakeouts-- connecting Headley to Denmark.

And acting on that information, the FBI arrested Headley at O'Hare International Airport on October 3, 2009.

>> I said, "Mr. Headley, we would like to do this "quietly and discreetly.

If you would just come with us."

And he said, "Very well."

>> ROTELLA: A fledgling plot against a newspaper had been stopped.

But for those killed ten months before in Mumbai, the evidence came too late.

It was a case study of the limitations of the most sophisticated spying capabilities.

>> Can you give me any examples where it has actually prevented a terror plot?

>> David Headley.

>> David Headley.

>> David Headley.

>> ROTELLA: U.S. officials later claimed Headley's case was an NSA success story.

>> In order to find the needle, we needed the haystack.

>> There is no other way that we know of to connect the dots.

>> ROTELLA: They said programs that collect phone records and overseas communications played a key role.

But others give a different account of the importance of the programs, like the one collecting overseas communications.

I don't want to either exaggerate or minimize the role that different agencies played.

The role that the NSA is playing in this case is support.

>> That is correct.

>> ROTELLA: So then this was not a plot though that is discovered by that program.

>> That's correct.

>> ROTELLA: After the Snowden revelations, a White House panel examined the intelligence community's claims about the Headley case and other terror investigations.

David Medine was its chairman.

>> In the Headley case, what we found was that the NSA did conduct a search of telephone records and provided the results of that search to the FBI.

But what we also found out was the information that was provided to the FBI either corroborated or duplicated information that the FBI already had.

>> ROTELLA: So what role did the program play in identifying or stopping Headley?

>> None.

>> ROTELLA: U.S. intelligence officials now concede that assertions about the NSA's role in stopping the Denmark plot were overstated.

But they insist that the agency did valuable work on the case.

>> Somewhere in those aircraft hangars full of haystacks, the Americans had some great stuff.

But only retrospectively are you able to find it.

>> I don't have any idea how the David Coleman Headley example justifies this dragnet approach.

First of all, they didn't stop him before he carried out significant operational activity for a long period of time.

Nobody detected him.

Nobody stopped him.

Lots and lots and lots of people died horrifically.

>> So I mean I don't know, but I want some...

I mean, it doesn't matter what I want, but I'm saying, I'd like some busts to happen.

>> ROTELLA: In the end, it wasn't high-tech spying that revealed the extent of Headley's role in the Mumbai attacks.

It was David Coleman Headley.

>> It was just a matter of moments, and he was ready to speak with us.

>> I was thinking about this, maybe he mentioned this, maybe he didn't or something.

>> We had a little bit of an idea that he was involved in Mumbai, but until we started talking to him and he gave it up, we just didn't know.

>> Go to pbs.org/frontline for more on the saga of David Coleman Headley.

>> ROTELLA: There had been plenty of signs that Headley was potentially dangerous.

>> You have plenty of evidence against me, but really... >> And check out the reporting of our partners at ProPublica on this story.

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