Two years before his death, an important guest would leave a tantalizing clue.

The visitors name: Cardinal Luigi dAragona.

He arrived with his private secretary, Antonio de Beatis.

De Beatis recounted the cardinal's visit in a detailed diary.

The aging Leonardo showed the cardinal several of his works... “St.

John the Baptist, ” “The Virgin and Child with St. Anne, ” and a portrait de Beatis describes as a “certain Florentine woman done from life.

” Nearly 400 years later, in 1905, the diaries were published.

They set off a storm of speculation about Leonardo's most famous work.

The diaries state that the Mona Lisa was commissioned by the late Giuliano de Medici.

This contradicts accounts that Leonardo had been hired by Lisa's husband, the silk merchant Francesco del Giocondo.

[speaking Italian] The two documents by Vasari and de Beatis differ with regard to the dates and the context.

They provide no clear answer.

But it is clear that Francesco del Giocondo and Giuliano de Medici cannot both be the client.

[speaking Italian] This supports the theory of two different portraits.

[Narrator:] For both accounts to be accurate, there would have to be two versions of the portrait: the finished masterpiece, painted for Giuliano de Medici, and an unfinished version, commissioned by Lisa's husband.

The tale of two Mona Lisas quickly spread throughout the art world, to people like dealer Hugh Blaker, who just a few years later, claims to have discovered his “Isleworth Mona Lisa.

” Had he perhaps not “discovered ” it at all?

Could he have painted it himself?

Or commissioned a talented forger to recreate the unfinished portrait?

Leonardo is probably the most copied artist of his generation, and it goes through the 16th century.

They keep making versions and copies and so on.

It clearly was a famous look.

You know, people liked pictures that said ‘Leonardo to them.

Hundreds of copies of the Mona Lisa circulate throughout the world, from different periods and of varying quality.

It's possible the Isleworth Mona Lisa is one of them.

As a forgery, its genius would lie in its obvious departures from the version in the Louvre.

No one would suspect it was an attempted copy.

It would be seen as an “additional ” Mona Lisa, one that had never been finished, as the Renaissance writer Vasari described.

The timing of her discovery is also dubious.

The Isleworth Mona Lisa appeared just after the original was looted from the Louvre, a time when a forger could be tempted to cash in on her absence.

So is the Isleworth Mona Lisa simply a fake?