In a stunning development in the investigation of the world's richest art heist, law enforcement officials said Monday they know who stole $500 million in master works from Boston's Isabella Stewart Gardner Museum and disclosed new detail about their interest in Hartford mobster Robert Gentile.

"The FBI believes with a high degree of confidence in the years after the theft the art was transported to connecticut and the Philadelphia region and some of the art was taken to Philadelphia where it was offered for sale by those responsible for the theft," said Richard DesLauriers, Special Agent in Charge of the FBI's Boston office. "With that same confidence we have identified the thieves who are members of a criminal organization with a base in the mid-Atlantic states and New England."

But since 2010, Gentile has been questioned repeatedly about his membership in the Boston branch of a Philadelphia-based criminal organization, as well as leads that place at least some of the stolen paintings in Connecticut and the Philadelphia area.