In their search, the authorities didn’t pay enough attention to Vincenzo Peruggia, an Italian laborer who created the protective glass around Leonardo da Vinci’s masterpiece. He was questioned twice, and let go.

Two years passed.

Mr. Peruggia then tried to sell the painting to Giovanni Poggi, the director of the Uffizi Gallery in Florence. Mr. Poggi immediately called the police.

The painting is now protected by bulletproof glass, and an alarm goes off if anyone tries to even touch the frame.

Kathleen Massara wrote today’s Back Story.

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