The FBI said it has identified the thieves who pulled off the 1990 theft of 13 artworks from Boston's Isabella Stewart Gardner Museum, which stands as the costliest art theft in U.S. history. The thefts are valued at $500 million.

"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.

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.