Vincenzo Peruggia, an Italian glazier, hid in a closet overnight to steal the portrait, later claiming he wanted to return the masterpiece to its native Italy.

The 1990 heist is the world's largest unsolved art theft. The frames remain empty because Isabella Stewart Gardner's will forbids any changes to her permanent collection.

Since 2004, the FBI Art Crime Team has recovered over 15,000 items valued at more than $800 million and maintains the National Stolen Art File.

The thieves left a note at the National Gallery reading "Thanks for the poor security," and the painting was recovered three months later in an undercover sting.