Background
In the early hours of 18 March 1990 guards at the Isabella Stewart Gardner Museum in Boston admitted two men posing as police officers. The men tied up the guards and took thirteen works of art, including Johannes Vermeer's 'The Concert' and Rembrandt's only seascape, 'The Storm on the Sea of Galilee'. The case is unsolved: no arrests have been made and no works recovered. The museum offers a $10 million reward for information, and the empty frames still hang on the walls.
Sources: Isabella Stewart Gardner Museum: The Theft · Wikipedia: Isabella Stewart Gardner Museum theft