>> NARRATOR: Tonight... >> A situation unfolding right now in Paris with a high number of casualties.

>> NARRATOR: Frontline and Pr oPublica investigate how ISIS and Al Qaeda brought terror to Europe.

>> Here, you have a convicted terrorist who's able to leave the country, go to a terrorist haven without being detected.

How is that possible?

>> NARRATOR: Propublica reporter Sebastian Rotella uncovers a trail of evidence across France, Belgium and Spain.

>> These individuals were on the radar.

>> NARRATOR: Revealing what Europe's top counter-terror officials knew.

>> NARRATOR: What they missed.

>> NARRATOR: And why Europe remains so vulnerable.

>> There's every reason to expect that we'll see ISIS lash out while it's under pressure in Syria and Iraq to maintain its relevance.

>> NARRATOR: Tonight on Fr ontline, "Terror in Europe."

>> NARRATOR: In early 2015, Belgian police, with the help of U.S. and French intelligence, were preparing to launch a raid on a terrorist cell thought to be on the verge of an attack.

The suspects were hiding out in the town of Verviers.

(gunfire) >> NARRATOR: When Belgian commandos stormed the hideout, they came under heavy fire.

(shouting and gunfire) They shot two men dead and wounded another.

Investigators found explosives, fake IDs, and police uniforms.

>> NARRATOR: Since January 2015, an unprecedented wave of terror attacks has overwhelmed Europe's defenses.

(gunfire) (man shouting) That month, attacks against Ch arlie Hebdo magazine and a Jewish supermarket in Paris left 17 people dead.

(gunfire) On November 13th in Paris, ISIS attacked multiple targets, killing 130 people.

Four months later... suicide bombings killed 32 in Brussels.

ProPublica reporter Sebastian Rotella has been covering terrorism for two decades.

Years before the attacks, he was already reporting on some of the jihadists who would go on to strike Europe, and the counterterror officials trying to stop them.

In this film, he sits down with the men and women on the inside of the fight against Al Qaeda and ISIS.

They reveal the missteps and systemic breakdowns that allowed known terrorists to hit the heart of Europe, how the problems persist today, and the unprecedented threat the continent faces.

>> These individuals were on the radar, they had traveled to Syria, they were known to law enforcement intelligence officials.

>> No system is perfect.

And we live in a free world.

How much of your freedom do you want to sacrifice for your security?

(gunfire, explosion) >> NARRATOR: In 2003, French intelligence began monitoring a group of Islamic radicals who lived near the Buttes-Chaumont Park in northeast Paris.

They were young, untrained and inexperienced.

But one member of this gang would ultimately carry out the Charlie Hebdo attacks 12 years later.

Cherif Kouachi was a petty criminal and aspiring rapper.

The son of Algerian immigrants, he'd grown up in an orphanage after his parents died.

When he was 21, Kouachi was radicalized by the U.S. invasion of Iraq and joined the extremists of the Buttes-Chaumont Gang.

They were plotting to go to Iraq and kill Americans.

>> NARRATOR: Louis Caprioli was then the counterterror chief of French domestic intelligence.

Rotella first met him while reporting on the Buttes-Chaumont Gang.

>> NARRATOR: Kouachi was sent to Fleury-Mérogis prison to await trial.

While he was locked up, his extremist connections only deepened.

The prison was a hotbed of jihadism dominated by Al Qaeda veterans.

Kouachi became friends with another radicalized criminal who would ultimately join him in his terrorist project: Amedy Coulibaly.

>> NARRATOR: Marc Trévidic was a top counterterror prosecutor and judge who investigated Kouachi's network.

He says the French judicial system was not set up to deal with the long-term threat they posed.

>> NARRATOR: As the gang had not carried out an actual attack, French law dictated that they be tried in a low-level court alongside robbers and drug dealers, where the maximum sentence was only ten years.

In 2008, Kouachi was convicted of recruiting fighters to go to Iraq and attempting to join Al Qaeda.

His sentence was three years, with 18 months suspended.

>> The contrast between the way European nations deal with terrorism from a criminal perspective and the United States is quite stark.

>> NARRATOR: Matt Olsen led the U.S. National Counterterrorism Center between 2011 and 2014; before that he was a prosecutor and the NSA's chief lawyer.

>> ROTELLA: What would the sentence be for somebody like that in the U.S.?

>> Hypothetically, someone like Kouachi with that charge would be looking at far in excess of 15 years, the conspiracy to provide support to a terrorism group.

And the important thing there is that 15 years, for somebody who's in their mid-20s or their 30s, you know, that brings them into their 40s or mid-40s, and the hope is that by the time they're released, they're not interested or too old to really be involved.

>> NARRATOR: Kouachi wasn't alone.

Amedy Coulibaly served only three years for his involvement in a plot to help a convicted terrorist escape from prison.

No one else in the Buttes- Chaumont crew served more than seven years, even dangerous fighters who saw combat in Iraq.

Today several are active terrorists.

>> NARRATOR: Having already spent 20 months in prison awaiting his trial, Kouachi left court a free man.

By this time, Louis Caprioli had retired.

>> ROTELLA: For a police officer like you, who has worked to put these people in jail, how does that make you feel?

>> NARRATOR: In the years following his release, Kouachi was investigated again for suspected terrorist activity, but never convicted.

In 2011, he took advantage of a fatal flaw in Europe's counterterror defenses: weak border control.

Concealing his identity by using his brother's passport, he left France and traveled to Yemen to join Al Qaeda.

>> ROTELLA: Here you have a convicted terrorist who is able to leave the country, go to a dangerous part of the world, a terrorist haven, without being detected.

How's that possible?

>> Well, absolutely, so somebody should not be able to cross international borders who's been convicted of a terrorism offense and who's seeking to travel to a place like Yemen.

Here in the United States after 9/11, we established a single watch list for known or suspected terrorists-- the no-fly list.

In Europe, there's not one single watch list for Europe.

They have not developed a way to effectively stop somebody from traveling, even though in this case, the individual was convicted of terrorist offenses.

>> NARRATOR: For more than a decade, counterterror chiefs have proposed laws to improve border defenses, such as giving European security forces systematic access to data that airlines collect about all passengers on the continent.

U.S. border guards have used this tool, known as passenger name record, or PNR, for 15 years.

But European politicians, concerned about privacy and data protection, repeatedly rejected PNR legislation.

>> Madam president of the council, read my lips: data protection directive.

>> We have, you know, maybe a different privacy mindset in certain countries in Europe compared to the U.S.

It depends on the different cultural, historical, political backgrounds of each country, and they are different.

>> NARRATOR: Rob Wainwright is the director of Europol, the agency tasked with coordinating law enforcement across the 28 countries of the European Union.

>> The privacy-security trade- off still goes back, I think, to the legacy from the Second World War where, you know, German and Austrian citizens are concerned about never again shall we arrive at a position where the State can have so much authority that they can collect unlimited amounts of personal data about their citizens.

And for good reason, actually.

>> NARRATOR: But counterterror chiefs say despite the concerns, PNR would help them intercept suspected terrorists.

>> NARRATOR: In Yemen, Cherif Kouachi met up with an old friend from the Buttes-Chaumont Gang.

Peter Cherif had himself absconded from France while on trial for terrorism charges and was now a fighter for Al Qaeda in the Arabian Peninsula, which was being monitored intensively by the U.S. >> The meeting that occurs in the summer of 2011, that's sort of the worst nightmare for intelligence services.

Because any time that Al Qaeda had access to someone from a Western European country like France, they would try to operationalize that person, that person could give them insights about Western culture, help them develop plans to carry out attacks.

>> NARRATOR: Peter Cherif arranged for Al Qaeda to give Kouachi terror training and money-- $20,000, according to U.S. intelligence.

A plot began to take shape.

>> NARRATOR: After three weeks in Yemen, Cherif Kouachi returned to France undetected.

He and his brother Said now became an Al Qaeda sleeper cell.

U.S. intelligence learned that one of the Kouachis had visited Yemen, and alerted the French, who started monitoring them.

But they discovered nothing of the Charlie Hebdo plot.

Surveillance resources were already stretched, and the number of European extremists was about to surge.

(explosion) >> NARRATOR: In 2012, the war in Syria dramatically changed the European security landscape.

(shouting) It spawned a new jihadist movement, ISIS, which set out to be even more brutal than Al Qaeda.

In 2014, the group formally split from Al Qaeda and declared a caliphate, or Islamic State, and summoned all Muslims to join them.

>> NARRATOR: Dolores Delgado is the chief counterterror prosecutor of Spain's high court.

As Spain's liaison to France and Belgium, she worked closely with her European counterparts to monitor the exploding numbers of ISIS recruits.

>> NARRATOR: Thousands of young European Muslims joined up.

Unlike recruits to Al Qaeda, aspiring ISIS militants often knew little about Islam.

>> NARRATOR: One country, Belgium, provided more ISIS militants for its size than any other in Europe.

Among them was a petty criminal named Abdelhamid Abaaoud.

The son of a shopkeeper from Brussels, Abaaoud had spent time in prison for assault and minor crimes.

As ISIS was emerging in Syria, Abaaoud began to draw on his criminal network to recruit volunteers to the cause.

>> NARRATOR: Alain Grignard is a senior counterterror officer with the Belgian federal police, an expert in Islamic extremism who speaks fluent Arabic.

His agency started tracking Abaaoud's network in 2013.

>> NARRATOR: Many of Abaaoud's recruits came from a single Brussels neighborhood: Molenbeek, the country's second-poorest district, with a large population of unemployed young people.

>> NARRATOR: Abaaoud was able to recruit his team with little interference from Belgian security forces.

For years, police and intelligence services in Belgium have been hampered by limited budgets, bureaucratic infighting, and weak laws for crime and terrorism.

One of Abaaoud's crew was a robber who had shot and wounded a police officer with an AK-47.

But he served only four years in prison.

>> Fundamentally some of the laws in Belgium seem quite out of date.

The counterterrorism infrastructure, as much as there are individuals with a great deal of experience, the infrastructure is not there in the laws and policies to support the type of response that's needed to take on the level of threat we see now.

>> NARRATOR: Abaaoud left Belgium to join ISIS in 2013.

Others from his Molenbeek crew soon followed.

In Syria, Abaaoud embraced the Islamic State's culture of extreme violence.

(shouting) >> NARRATOR: Then, in August 2014, Western powers began to bomb ISIS strongholds.

(shouting) >> NARRATOR: In late 2014, ISIS began to create an external operations unit.

It developed a plan for a campaign of revenge attacks against Europe.

The unit would deploy as many as 200 terrorist operatives to launch attacks in their home countries.

At the same time, hundreds more fighters were returning to Europe without specific terrorist missions, and hundreds of Europeans had been radicalized at home without visiting Syria.

Any one of them could have posed a threat.

>> You know, 5,000 European nationals have gone out to Syria and Iraq.

We don't know exactly how many have come back, we kind of figure around a third of that, so, you know, 1,500 or more.

So it's extremely difficult, I think, to get it right in terms of who do we monitor and how do we monitor them.

Because we don't have the resources or indeed the culture in our society to put 24/7 surveillance on thousands of citizens every day.

>> NARRATOR: To make matters worse, the 28 countries of the European Union are often wary about sharing intelligence with each other.

>> NARRATOR: As more and more ISIS recruits returned to Europe, the authorities in France and Belgium were overwhelmed.

>> NARRATOR: In June 2014, French spy chiefs made a fateful decision.

For the last three years, they had been monitoring Al Qaeda veterans of the Buttes-Chaumont Gang: Cherif Kouachi, and his brother Said.

Meanwhile, Kouachi's old associate Amedy Coulibaly had just been released from prison.

French domestic intelligence now decided to stop watching them and shift surveillance resources onto the growing threat from ISIS.

(man shouting) (gunfire) >> Breaking news out of France.

The French police right now are hunting for masked gunmen who stormed the offices of the satirical newspaper, opened fire in the French ca pital today.

At least 12 people are dead, four others are in critical condition.

>> NARRATOR: 12 people were shot dead in the Charlie Hebdo attacks.

Within hours, the names of the killers surfaced.

For the former counterterror chief Louis Caprioli, it was unsettling news.

>> NARRATOR: After a massive manhunt, the Kouachi brothers were killed in a shootout with French police.

(gunfire) They declared allegiance to Al Qaeda shortly before they died.

That same day, their friend Amedy Coulibaly carried out his part of the plot, shooting four people dead in a Jewish supermarket before being killed by a SWAT team.

Although he had been radicalized by Al Qaeda, he claimed allegiance to ISIS.

>> NARRATOR: As France reeled from the Charlie Hebdo attacks, another plot was being uncovered across the border in Belgium.

(shouting, gunfire) On the 15th of January, Belgian police raided a house in the town of Verviers, killing two terrorist suspects and wounding one.

U.S. and French spy agencies had helped track their return from Syria.

Investigators found evidence that they were part of an ISIS cell deployed by the young Belgian extremist Abdelhamid Abaaoud.

>> Because of the Verviers case, Abaaoud came very clearly in the picture.

>> NARRATOR: Eric Van der Sypt is a Belgian counterterror prosecutor.

>> We realized that he was active in the recruitment of people from France, from Belgium and he trained them, and he was responsible for sending back people to Western Europe also.

People that want to die, they're not afraid to die, and they're not afraid to kill other people while doing so.

>> NARRATOR: Abaaoud now became the subject of an international manhunt.

European and U.S. intelligence detected his cell phone in Athens, but by the time Greek police raided his safe house, he had already fled.

It was the first of what would be several missed opportunities to capture him.

>> Abaaoud disappeared, and it's a pity we lost him because we knew who he was, what he was doing, and sure, we would have loved to have captured him, it's a sure thing, but he got away.

He managed to escape, and he managed to go back to Syria.

>> NARRATOR: Over the next few months, ISIS sent a series of lone operatives to attack Europe.

Authorities suspected Abaaoud was involved.

Then in June 2015, Spanish counterterror officials made a breakthrough.

With the help of U.S. intelligence, they detected an alleged ISIS fighter who had just returned to Europe from Syria via Poland.

The Poles arrested and questioned him, along with Spanish investigators.

His name was Abdeljail Ait El Kaid.

>> NARRATOR: Ait El Kaid admitted he'd been sent to Europe to commit an attack.

>> NARRATOR: Ait El Kaid gave up the name of another suspected operative sent by Abaaoud to hit European targets.

In August, the man was arrested by French police when he returned from Syria.

His name was Reda Hame.

Marc Trévidic, who was weeks away from the end of his term as France's top counterterror magistrate, now questioned Hame.

>> NARRATOR: On August 21, just one week later, the warnings were confirmed.

A heavily armed man, Ayoub El Khazzani, opened fire on a high-speed train between Amsterdam and Paris.

The gunman was overpowered by three American tourists, and no one was killed.

A year earlier, Spanish authorities had actually warned their French counterparts about Khazzani, who they'd tracked from Spain to France.

But investigators believe he eventually made his way to Syria, where he was allegedly trained and sent back to Europe by Abaaoud.

>> NARRATOR: Abaaoud was now wanted across the continent.

>> NARRATOR: In fact, by September, Abaaoud was already back in Europe laying the groundwork for the most ambitious plot yet.

Investigators say he slipped in through Greece with the help of smugglers.

Policing of Europe's external borders is left largely to individual nations, whose budgets and capabilities vary.

>> NARRATOR: Abaaoud then took advantage of Europe's open internal borders to travel freely from country to country.

>> NARRATOR: As the war in Syria intensified in 2015, Germany declared it would welcome all refugees fleeing the conflict.

Hundreds of thousands of refugees and migrants flooded into Europe, but there was no comprehensive system to vet them.

Most came in through Greece, which was overwhelmed, and it could only thoroughly screen about a third of the arrivals.

Counterterror chiefs say they warned about terrorists exploiting this opportunity.

>> NARRATOR: Jean-Louis Bruguière was a top French counterterrorism judge for two decades.

He says the migrant crisis exposed the dysfunction of the European Union's approach to security.

>> To be fair, at the time, the sort of mood within Germany, the mood around many countries in Europe was one of, we're living through a great humanitarian tragedy.

These desperately poor afflicted people needed a place of refuge, and it was Europe's job to provide that.

Now we're saying, you know, there's been some dreadful terrorist attacks and on reflection maybe that wasn't the right thing to do.

Well, you know, that's a little harsh.

>> NARRATOR: Abaaoud now used the chaos of the refugee flow to get a team of militants into and around Europe.

Two of the team entered Europe with refugees on the island of Leros.

One used a stolen Syrian passport flagged by Interpol as possibly being used by terrorists.

But Greek authorities weren't checking Interpol's database regularly, and the men were let through.

Bomb-maker Najim Laachraoui and one of the leaders of the cell, Mohamed Belkaid, are also thought to have entered Europe through Greece.

>> They both had false Belgian identity papers.

It's a tool they used.

I think they used the fact that there were a lot of refugees at that time over there so they could blend in, to stay anonymous.

(men shouting) >> NARRATOR: On September 9, Laachraoui and Belkaid were met at the Budapest train station, then packed with refugees, by another suspected member of the cell: Salah Abdeslam.

Hours later, police stopped their Mercedes at the Austria-Hungary border.

They were known extremists.

One was wanted on a terrorism warrant.

One was on an EU watch list.

But the police didn't spot anything suspicious during questioning or in their databases, and the car was waved on.

>> ROTELLA: Why were they let through without further investigation?

>> It's very easy after the event to say, "Well, we should have got this guy because he was on the record."

We're dealing with, you know, 20-plus countries in Europe sharing a different set of information systems, not all of them sort of interconnected, not all of them holding sensitive terrorist data.

Not all the intelligence is shared with all of the partners on all of the systems at the same time, and we have a challenge, I think, in Europe where we have different information systems in different parts, in different countries, but also in different parts of the EU architecture that are not hooked up.

>> NARRATOR: By the end of October, Abaaoud had everything in place: weapons, explosives, targets and the men to hit them.

(gunfire) >> We're coming on the air to tell you about a situation unfolding right now in Paris, where there have been a number of apparent attacks.

>> ...three people are dead in multiple attacks across the French capital.

There were at least six sh ootings in various locations.

And at this moment, police are storming a concert hall.

>> I remember saying to myself, "I hope there is not a Belgian connection."

And I was proven wrong the following hours already, and the Belgian connection, if I may call it like that, was soon very, very clear.

>> NARRATOR: 130 people were killed in the attacks.

Most of the suspected plotters were already known to the authorities, and multiple opportunities to stop them had been missed.

At least six were wanted on international arrest warrants for terrorism.

One was under police surveillance with wiretaps and a hidden camera.

At least seven were on terrorist watch lists.

At least 12 were stopped, questioned, and even arrested as they traveled around Europe and back and forth to Syria to prepare the Paris plot.

>> These individuals were on the radar.

They had traveled to Syria.

They were known to law enforcement intelligence officials.

Even with that information in the hands of intelligence and law enforcement, they were able to really carry out large-scale, spectacular, catastrophic-style attacks.

Because law enforcement and border patrol officials from one country simply don't communicate with their counterparts in another country in a way that would make information that they possess actionable and really disrupt or stop a terrorist from moving across their border.

>> NARRATOR: The Paris attacks were staged almost entirely from Belgium.

That's where the bombs were made, where coordinators directed the attacks by phone.

French security chiefs say the Belgians should have done more to stop the plot.

>> NARRATOR: Five days after the attacks, Abaaoud was tracked down to an apartment on the outskirts of Paris.

(gunfire) In the battle that followed, one of the plotters detonated a suicide vest.

(explosion) Abaaoud's remains were identified two days later.

But then investigators discovered that other suspected leaders of the cell were still on the loose.

>> NARRATOR: Seven remaining suspects were holed up in safe houses back in their old neighborhoods in Brussels, where they were working on a new plot.

Unknown to Belgian intelligence and the NSA, which was helping hunt the fugitives, the bomb maker, Najim Laachraoui, was in direct contact with a shadowy ISIS chief in Syria known as Abu Ahmad.

Abu Ahmad avoided interception by using encrypted communications to give detailed orders and bomb-making instructions.

>> The individuals were sharing information, they were getting instruction on how to make explosives from individuals in Syria.

The content of those communications were encrypted.

There's no technological way to intercept those communications.

We have not solved this problem.

This is a problem that is with us today.

>> NARRATOR: Investigators say Abu Ahmad worked closely with another senior ISIS operative in Syria known as Abu Sulaiyman al Fransi.

U.S. counterterror officials believe he is a 26-year-old Moroccan immigrant who grew up in France, served in the French Foreign Legion, and did prison time for drug dealing before joining ISIS.

He is now suspected of playing a lead role in overseeing the Paris and Brussels plots.

>> It shows a level of direction from ISIS, you know, that this is not simply an attack that was inspired by ISIS propaganda or online communication; this is an attack that was actually being directed at a degree of specificity by ISIS central, ISIS leadership.

>> NARRATOR: In March, after a four-month hunt, Belgian police discovered a series of apartments rented using false identities and finally closed in on the cell.

They shot dead one of the suspects and captured another: Salah Abdeslam.

But the bomb maker and others were hiding elsewhere.

>> Breaking news right now: tw o explosions rocking the main terminal at Brussels Airport.

There are reports of another attack, an explosion at a subway, a metro station.

>> We should underscore that this appears to be a coordinated attack.

>> NARRATOR: 32 people died in the Brussels bombings.

ISIS has vowed more attacks in Europe even as they lose ground in Syria and Iraq.

>> NARRATOR: In response, European leaders have set up a new Counterterrorism Center and recently approved the passenger name record to bolster border security across the continent.

But the threat has worsened the political crisis for the European Union.

In June, the United Kingdom voted in a referendum to leave the EU.

Other countries are considering doing the same.

The counterterror chiefs say the systemic problems remain and Europe is as vulnerable as ever.

>> Go to pbs.org/frontline for more of our partner ProPublica's reporting on terror in Europe.

>> ROTELLA: Why were they let through without further investigation?

>> They have not developed a way to effectively stop somebody from traveling.

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