Security guard Paul Daley stands guard at the door of the Dutch Room of the Isabella Stewart Gardner Museum in Boston, March 21, 1990. Jim Bourg/Reuters

There have been "confirmed sightings" of priceless artworks stolen in a 1990 heist that remains an enduring mystery, FBI sources in Boston told ABC News.

The theft of 13 masterpieces from the Isabella Stewart Gardner Art Museum in Boston is still the largest property crime in U.S. history. It was a quarter century ago, but the FBI said the trail has not gone cold.

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Though no one has seen all 13 works together investigators said there have been confirmed sightings of some of the paintings from sources deemed credible. The last known sighting was in or around Philadelphia.

Additionally, the FBI has narrowed a list of suspects, that at the outset was pages long, to just a handful of figures tied to organized crime: Carmello Merlino, Robert Guarente and Robert Gentile, a member of Philadelphia La Cosa Nostra. He is the only one of the three still living.

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Gentile has claimed to have no knowledge of the Gardner heist or the stolen artwork, but law enforcement source told ABC News his most recent polygraph test was "off the charts" deceptive.

A search warrant executed at his house in Manchester, Conn., in 2012 turned up no art, but investigators found it "puzzling" Gentile had police paraphernalia. The items were similar to those used in the 1990 heist, during which thieves posed as Boston police officers. At the moment there was not enough evidence against Gentile to arrest him for the theft. As one FBI source put it, "He won't cooperate. It is cagey for an ex-con to have [paraphernalia] but not illegal."

On March 18, 1990, two men disguised as police officers tied up the security guards at the museum and stole rare paintings by Rembrandt, Degas, and Vermeer and others valued at approximately a half- billion dollars.

The museum has offered a $5 million reward for the return of the stolen works.