It turned out that Louvre employee Vincenzo Peruggia stole the Mona Lisa by simply walking out the door with it hidden under his coat.

The theft was master-minded by a con-man who had commissioned a French art forger to make copies of the painting so he could sell them as the missing original. Because he didn't need the original for his con, he never contacted Peruggia again after the crime. After having kept the painting in his apartment for two years, Peruggia grew impatient and was finally caught when he attempted to sell it to a Florence art dealer.