Robert Gentile, a reputed Connecticut mobster, is wheeled into the federal courthouse in Hartford, Conn., Tuesday, Sept. 5, 2017. Gentile, a reputed Connecticut mobster who authorities say is the last surviving person of interest in the largest art heist in history, could not remember pleading guilty in an unrelated weapons case, his lawyer said, prompting a judge to delay his sentencing Tuesday. (Patrick Raycraft/The Courant via AP) Credit: Patrick Raycraft | AP

An octogenarian accused mobster, who prosecutors believe may hold some of the last remaining clues needed to solve the largest art heist in U.S. history, is due to be sentenced on Tuesday for illegally selling guns.

Robert Gentile admitted in April to illegally selling a loaded firearm to a convicted killer, the result of what his lawyer calls a Federal Bureau of Investigation sting operation aimed at pressuring him into providing details on paintings stolen from Boston’s Isabella Stewart Gardner Museum in March 1990.

Gentile, 81, has repeatedly denied knowing the whereabouts of any of the art, valued at an estimated $500 million, taken in one of the longest unsolved high-profile crimes in Boston, and did not address the matter during the hearing.

But during a polygraph test performed as part of the Gardner investigation, Gentile had an intense reaction when he was shown images of the missing paintings, while he remained calm when shown unrelated artwork, according to a law enforcement source briefed on the test.

Gentile could be sentenced to almost six years in prison when he appears in U.S. District Court in Hartford, Connecticut on Tuesday. It is unclear how long a sentence prosecutors are actually seeking as court papers spelling out the request were filed under seal.

The Gardner heist was carried out by two men dressed in police uniforms who apparently overpowered a night security guard who had buzzed them in.

None of the 13 stolen artworks, which include Rembrandt’s “Storm on the Sea of Galilee,” and Vermeer’s “The Concert,” has been recovered.

At a 2015 hearing, prosecutors said Gentile was secretly recorded telling an undercover FBI agent that he had access to at least two of the paintings and could sell them for $500,000 each.

A 2012 search by the FBI of Gentile’s home turned up a handwritten list of the stolen art, its estimated value and police uniforms, according to court documents.