HARTFORD (AP) - 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.

Robert Gentile appeared for sentencing in the firearms case in federal court in Hartford. But Judge Robert Chatigny said he could not go forward with sentencing because of mental health concerns raised by Gentile's lawyer.

Federal prosecutors have said they believe the 81-year-old Manchester resident has information about the still-unsolved 1990 heist at the Isabella Stewart Gardner Museum in Boston. Thieves stole an estimated $500 million worth of artwork, including works by Rembrandt, Edouard Manet, Edgar Degas and Johannes Vermeer.

The paintings were stolen on March 18, 1990. Two thieves dressed as police officers walked into the museum, bound and gagged two guards and removed a number of masterworks, cutting some of the largest pieces from their frames.

Thirteen works of art were stolen, including Rembrandt's only known seascape, "Christ in the Storm on the Sea of Galilee, and Vermeer's "The Concert."

Gentile has denied knowing anything about the theft. The artwork remains missing, and no one has been charged.

Gentile's attorney, A. Ryan McGuigan, told Chatigny on Tuesday that Gentile could not remember pleading guilty to firearms charges in April, and he was having difficulty communicating with Gentile.

Assistant U.S. Attorney John Durham suggested Gentile might be faking symptoms to avoid being sentenced. Durham said Gentile has shown no signs of incompetency while detained at the Bridgeport Correctional Center during the past several months.

Sentencing was delayed for about two months to provide time to determine whether Gentile needs to undergo another competency evaluation. He previously was ruled competent.

"I think the court is not supposed to sentence a person who doesn't understand why he is being sentenced," Chatigny said.

The plea deal calls for Gentile to serve three to six years in prison.

Federal agents have searched Gentile's home several times in what McGuigan believes were attempts to find some of the stolen paintings, or evidence connected to the heist. Last year, authorities said they seized numerous firearms and ammunition at Gentile's home.

In 2013, Gentile was sentenced to 2½ years in prison in a weapons and prescription drugs case that first revealed federal authorities' belief that he knew something about the heist.

Prosecutors have said another gangster's widow claimed her husband gave Gentile two of the paintings. Authorities also have said Gentile talked about the stolen paintings with fellow prisoners and once told an undercover FBI agent he had access to two of the paintings and could negotiate the sale of each for $500,000.

The Boston museum is offering a $10 million reward for information that leads to the recovery of the paintings.

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