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BOSTON (Reuters) - An octogenerian alleged mobster who police believe may be able to help solve the largest art heist in U.S. history will undergo a psychiatric evaluation before being sentenced on gun charges, a federal court judge in Connecticut ruled on Friday.

The decision came after Robert Gentile, 81, appeared in court earlier this month for sentencing on charges he pleaded guilty to in April, but claimed to have no memory of entering a plea or of the events involved.

“The defendant’s counsel has become increasingly concerned about the defendant’s ability to understand the charges against him and assist in his defense,” defense attorney Ryan McGuigan wrote in a motion asking for the competency hearing. “Counsel has also observed that the defendant has been getting dates and facts significantly confused.”

U.S. District Judge Robert Chatigny granted the request without comment on Friday.

Gentile in April had pleaded guilty to charges of illegally selling guns to a felon. His attorney said the charges resulted from a sting operation aimed at pressuring him into providing details on 13 pieces of art stolen in one of the longest unsolved high-profile crimes in Boston’s history.

Some $500 million worth of art, including Rembrandt’s “Storm on the Sea of Galilee” and Vermeer’s “The Concert,” was stolen from Boston’s Isabella Stewart Gardner Museum in March 1990.

Gentile has repeatedly denied knowing the whereabouts of any of the art.

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.