Shortly after midnight on March 18, 1990, two men broke into the Isabella Stewart Gardner Museum in Boston and committed the largest art heist in history. They stole a dozen masterpieces, including one Vermeer, three Rembrandts, and five Degas. But after thousands of leads--and a $5 million reward--none of the paintings have been recovered. Worth as much as $500 million, the missing masterpieces have become one of the nation's most extraordinary unsolved mysteries.

After the death of famed art detective Harold Smith, reporter Ulrich Boser decided to take up the case. Exploring...