The theft of $500 million worth of masterpieces from Boston's Isabella Stewart Gardner Museum nearly a quarter century ago is not a cold case, says the FBI. An agent in charge of the investigation for the bureau says there are confirmed sightings of the missing artwork from credible sources.

FBI Special Agent Geoff Kelly told MyFoxBoston.com, "We believe that over certain periods of time, this artwork has been spotted."

"There have been sightings of it, confirmed sightings," said Kelly. 

There are three persons of interest who are connected to organzied crime, says Kelly. Two have died, leaving Robert Gentile, 78, who has denied any knowledge of the missing work. His home in Manchester, Conn., was searched in 2012 and nothing was found.

On March 18, 1990, two men, described as white and in their 30s, dressed a policeman and told the museum's security guards that they were responding to a report of a disturbance. The suspects tied up the guards and, in 81 minutes, took 13 artworks, including Rembrandt's "The Storm on the Sea of Galilee" and Vermeer's "The Concert." 

Two FBI informants told law enforcement that Carmello Merlino was planning to return Rembrandt's “The Storm on the Sea of Galilee” in the 1990s, to try to collect reward money. His plans were foiled when he was arrested for an aborted armored car heist, according to MyFoxBoston.com.

Kelly thinks that the late Robert Guarente, one of the persons of interest, then passed control of the stolen Gardner artwork to Gentile.

Since "there is no market" for the stolen art, authorities believe it is hidden away. The trail is not cold.

 Anyone with information on the Gardner heist is urged to call the FBI at 617-742-5533.