HARTFORD, Conn. (Reuters) - A federal judge on Tuesday delayed sentencing an accused octogenarian mobster who authorities believe may hold clues needed to solve the largest art heist in U.S. history after he claimed to not recall pleading guilty to illegally selling guns.

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U.S. District Judge Robert Chatigny in Hartford, Connecticut called off sentencing Robert Gentile, 81, after a defense lawyer said he could not remember pleading guilty in April to illegally selling a loaded firearm to a convicted killer.

“This morning he was in a fog,” said defense lawyer Ryan McGuigan.

The plea came in a case that stemmed from what McGuigan has called a Federal Bureau of Investigation sting operation aimed at pressuring Gentile into providing details on paintings stolen from Boston’s Isabella Stewart Gardner Museum in March 1990.

Chatigny’s decision to delay sentencing came over the objection of Assistant U.S. Attorney John Durham, who countered it was “much more likely he doesn’t want to confront sentencing.”

Durham said that on an Aug. 10 monitored jailhouse phone call with his wife, Gentile was “completely oriented” and understood he was going to be sentenced.

Chatigny said he understood “the government’s position that Mr. Gentile is feigning difficulties.” But Chatigny said he was required to delay sentencing given what McGuigan said. He ordered further briefing on Gentile’s competency.

Gentile 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. He did not address the matter while pleading guilty.

During a polygraph test performed amid the Gardner probe, 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.

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, court documents state.