"We have identified the thieves who are members of a criminal organization with a base in the mid-Atlantic states and New England," Richard DesLauriers, special agent in charge of the FBI's Boston office, said in a statement.
The frames that once held works including Rembrandt's "Storm on the Sea of Galilee" and Edouard Manet's "Chez Tortoni," now hang empty on the museum's walls -- in keeping with the precise instructions on the museum's appearance that its founder left in her will.
On the night of March 18, 1990, two men dressed as police officers arrived at the private museum's front door and a security guard let them in. The thieves allegedly overpowered both guards, who were found duct-taped to chairs in the museum's basement the next morning.
The 13 stolen artworks included paintings, drawings, sculpture and a beaker.
The Gardner Museum was founded by Isabella Stewart Gardner, an art collector who died in 1924. Her will contained very specific conditions on the running of the museum, including the arrangement of her collection and free admission to anyone named Isabella, a practice that continues today.
The FBI solved Boston's other long-running crime mystery in June 2011, when it found accused mobster James "Whitey" Bulger hiding in a seaside California community. Bulger, who is accused of committing or ordering 19 murders, was arrested on a tip that came in after the FBI launched a publicity campaign aimed at tracking him down; he had been on the run since 1994.