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(CNN) - The FBI said Monday it believes it knows who was behind one of the most significant art heists in the United States - the 1990 theft of 13 precious works, once valued at $500 million, from Boston's Isabella Stewart Gardner Museum.
A couple of catches in the announcement: The FBI didn't reveal the suspects' names, and the artwork still hasn't been recovered. But the FBI said the suspects "are members of a criminal organization with a base in the mid-Atlantic states and New England."
The bureau also said it believes the artwork - including paintings by Rembrandt and Vermeer - was taken to Connecticut and the Philadelphia area, and that the thieves unsuccessfully tried to sell some of the artwork in Philadelphia about 10 years ago.
"After the attempted sale ... the FBI's knowledge of the art's whereabouts is limited," the FBI said in a statement released Monday.
The announcement comes on the 23rd anniversary of the theft, which the FBI says is one of the largest property crimes in U.S. history.
The FBI says it's releasing the new details of the probe in part to remind people of a $5 million reward for information leading to the recovery of the items in good condition.
CNN's Carol Cratty, Devon Sayers, Jason Hanna and Susan Chun contributed to this report.