(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 big catches in the announcement: The FBI didn't reveal the suspects' names, said that the statute of limitations has run out so it can't charge anyone with the theft and that the artwork still hasn't been recovered.
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.
Charges still could be brought in the future against anyone holding the stolen masters. But authorities suggested the possibility of immunity would be considered should someone involved in keeping the art hidden away come forward and help the FBI to recover it.
"After the attempted sale ... the FBI's knowledge of the art's whereabouts is limited," the FBI said in a statement released Monday. At a Boston news conference, reporters pressed the FBI to name the suspected thieves, but officials declined, saying doing so might harm the ongoing investigation.
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.
In March 1990 two men posing as Boston police officers entered the museum in the middle of the night and tied up two watchmen. Among the the stolen items was a painting called "The Concert" by Johannes Vermeer, one of only 34 known paintings by the Dutch artist. Also pilfered were works by Rembrandt, Manet and Degas. Many view this as the single greatest art heist of all time.
U.S. Attorney Carmen Ortiz said she has visited the Dutch Room at the museum several times and has seen the empty frames where the paintings once hung. "I was reminded of the enormous impact of this theft," said Ortiz. "I do remain optimistic that one day soon the paintings will be returned to their rightful place."
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. The FBI's publicity effort will include a website devoted to the Gardner Museum robbery and the use of social networks and advertising on digital billboards in the Philadelphia region. Anyone with tip may call the FBI at 800-CALL-FBI or contact the agency online at https://tips.fbi.gov.
CNN's Devon Sayers and Susan Chun contributed to this report.