Paintings and drawings stolen in the largest art heist in U.S. history passed through the Philadelphia area, the FBI says. On the 23rd anniversary of the very cold case, the FBI said it had identified the two thieves and asked for the public's help in locating half a billion dollars worth of stolen goods.

Frames have hung empty in the Isabella Stewart Gardner museum in Boston since 1990. The works they used to hold include pieces by European masters Degas, Manet, Rembrandt and Vermeer. Two men dressed as Boston police officers handcuffed security guards and took their time removing the works of art.

The FBI says the paintings have been offered for sale in Philadelphia and Connecticut.

"What the FBI is doing is moving the spotlight which had really been shined (sic) for the past two decades on the Boston area and moving it southwards towards Philadelphia and Connecticut," noted Ulrich Boser, who wrote a book on the famous art heist.

"What we see here is much more convincing evidence that the paintings have moved from the Boston area," Boser said.

No paintings turn up in Connecticut raid

Last year, the FBI raided a house in Connecticut belonging to a man believed to be involved with the theft. Two searches of his home revealed no paintings.

In search of more information, the FBI will put up billboards of the paintings in the region. Experts such as Ulrich believe all the stolen art is still together but the crime is beyond the statute of limitations.

The bizarre case has been one big art mystery for two decades, rife with explanations ranging from the more probable mob involvement to a conspiracy theory that held Harvard University responsible for the theft.

Ulrich explains the strict instructions left by the museum's eccentric founder, Isabella Stewart Gardner, ordered that if anything were moved or changed, the whole collection should go to Harvard.

The art world hopes the latest news means that the paintings have, at least, survived. The $5 million reward the museum is offering may hasten their return.