The police photograph and fingerprint record from the arrest of Vincenzo Peruggia. Courtesy of Joe Medeiros

For exactly a century, mystery has wrapped the most famous art crime in history — the theft of the Mona Lisa.

What many to consider the greatest portrait of all time, painted by Leonardo da Vinci from 1503 to 1507, disappeared from the Louvre on August 21, 1911. It was stolen by Vincenzo Peruggia (1881-1925), an Italian immigrant who lived in Paris with the masterpiece for over two years.

Peruggia was never apprehended until he returned the Mona Lisa to Florence through an Italian art dealer, claiming he stole the painting to return it patriotically to the Italian people.

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However, the case has remained as elusive as the Mona Lisa's smile.

It was hard to believe that Peruggia committed the theft alone, and several conspiracy theories arose.

"The prevailing theory was that he was just a small cog in a grand scheme to sell Mona Lisa forgeries to American millionaires. The theft of the real Mona Lisa was the only way to convince the buyers they were purchasing the real thing," Joe Medeiros, author of the 88-minute documentary "The Missing Piece: The Truth About the Man Who Stole the Mona Lisa," told Discovery News.

Medeiros, the former head writer for "The Tonight Show with Jay Leno," acquired copies of 1500 documents in the French an Italian archives, including police files and court documents, and finally discovered that money -– not really patriotism –- lay behind the famous theft.

Vincenzo Peruggia. Italy State Police/Wikimedia

In an attempt to find clues about Peruggia the man — who he was, what he thought and why he stole the painting — Medeiros met with Peruggia's daughter Celestina in Italy.

But Celestina, who passed away in March at 87, knew very little about father.

"He died when she was a toddler," said Medeiros.

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The filmmaker went to the Louvre and re-traced the route Perruggia took to steal the painting.

At the time of the theft, Peruggia was a 29 year-old housepainter who had worked at the Louvre for a short time helping cover 1600 masterpieces with glass to protect them from vandalism.

Peruggia became familiar with all the Italian art and wondered why it was in a French museum.

He read that Napoleon had looted Italy's art treasures when he conquered the country and brought them back to Paris. Thus he believed that all the Italian art in the Louvre was there illegally and decided to bring one picture back to its country.

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Unaware that the Mona Lisa was sold by Leonardo da Vinci himself to King Francois I of France, he turned to this painting because it was small and easy to carry.

"He stole the masterpiece by simply walking into the museum on a Monday when the Louvre was closed for cleaning. He was dressed in a white smock and thus blended in with he other workers," said Medeiros.

It was the easiest task: Peruggia removed the painting from the wall, took it from its frame and walked out of the museum with the Mona Lisa under his arm, wrapped in his smock.

The theft wasn't discovered until the next day because the Louvre guards assumed the masterpiece was with the museum photographer.

Peruggia at the trial in Florence. Wikimedia

Only when the painting's empty frame was found on a service stairwell, did Louvre officials began to suspect the worst.

Some 60 detectives swept through the museum. Despite the fact that Peruggia left generous fingerprints on the glass that covered the painting, the lead was not followed up.

The theft was a sensational event, and thousands flocked to see the empty space between Titian's "Allegory of Alfonso d'Avalos" and Correggio's "Mystical Marriage" in the Salon Carré.

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It was assumed that some genial mastermind lay behind the theft: even Pablo Picasso and his friend, the poet and art critic Guillaume Apollinaire were questioned as suspects in the theft (they both had previously purchased several Iberian statuettes stolen from the Louvre's archaeological collections.)

"The 'Mona Lisa' was only two miles from the Louvre in the tiny room of Vincenzo Peruggia. He lived there with the masterpiece for nearly 2 and half years," Medeiros said.

Then in December 1913, Peruggia brought the painting to an art dealer in Florence, claiming to be an Italian patriot.

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He had hoped to be rewarded by the Italian government for his efforts. But instead of being compensated, he was jailed.

However, the court gave him a lenient sentence: overrall, he served a little over 7 months.

Peruggia's letters to his parents as well as a 61-page report done by a court-appointed psychiatrist, provided Medeiros with the final clues of the man's true motive for stealing the 'Mona Lisa'.

"It wasn't what his daughter Celestina would want to hear," Medeiros said.

According to the filmmaker, Peruggia committed the theft alone and for his own reason — and it wasn't patriotism.

"He was hoping to make his fortune. He speaks nothing of wanting to help his country, but writes he will be doing something to bring a better life to all of his family," said Medeiros.

The documents reveal that Peruggia was tired of his painting job because it was making him physically ill (he had lead poisoning.)

He no longer wanted to travel to Paris to find work because as an immigrant he was poorly treated by his co-workers. And he was a lonely man tired of being away from his family.

"To him, stealing the Mona Lisa and returning it to Italy was his ticket out of this life into a better one. It didn't turn out that way," Medeiros said.

While seeking worldwide distribution, "The Missing Piece" will be shown at two test screenings in Philadelphia on August 21st and August 22nd.