FBI posters displaying works by artists Johannes Vermeer and Edgar Degas are seen during a press conference held to appeal to the public for help in returning artwork stolen in 1990 from the Isabella Stewart Gardner Museum in Boston, Massachusetts March 18, 2013. The FBI is offering a 5 million dollar reward for the return of the paintings. REUTERS/Jessica Rinaldi

The FBI appears closer to solving an art theft mystery that has left investigators scratching their heads for nearly a quarter century.

An agent in charge of the recovery operation said his team has confirmed sightings of some of the art -- worth $500 million -- stolen from the Isabella Stewart Gardner Museum in Boston in the years after the theft. Up until now, there were no official sightings of the artwork.

In 1990, two men disguised themselves as police officers, entered the Gardner Museum, subdued the guards and stole 13 pieces of art. They knew what they were doing and made off with three Rembrandts, a Vermeer and more. Despite a $5 million reward and guaranteed immunity from federal prosecutors, the works haven’t been seen since.

The confirmed sightings corroborate a series of tips that led investigators to three men and a trail going from Boston to Connecticut and Philadelphia, where they were put up for sale. The men, Carmello Merlino, Robert Guarente and Robert Gentile, all had connections to organized crime. Merlino apparently had plans to return a Rembrandt to win the award in the late 1990s, but he was arrested in an unrelated armored car robbery and the painting remains at large.

Only one man, Gentile, is still alive. In 2013, federal agents searched his home in Connecticut but only found police equipment that could be linked to the heist. Gentile’s lawyer says Gentile had nothing to do with the Gardner heist.

FBI Special Agent Geoff Kelly told FOX 25 Boston that there’s good reason to believe the paintings are still in fair condition. He says stolen artwork can be used as a bargaining chip if a thief is arrested. Kelly said that the paintings had been seen as recently as "a dozen years ago."

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Watch an excellent summary of the heist and the thieves with former Boston Globe reporter Stephen Kurkjian here.