HARTFORD, Connecticut (AP) -- Federal authorities returned on Monday to search the Connecticut property of a reputed mobster suspecting of having knowledge about the largest art heist in U.S. history.

FBI agents went to the Manchester home of Robert Gentile. It wasn’t immediately clear why they were there. Agents sent up a small tent in the front yard.

Federal prosecutors believe the 79-year-old Gentile knows about the still-unsolved 1990 theft at the Isabella Stewart Gardner Museum in Boston. Thirteen pieces of art worth an estimated $500 million were stolen and never recovered, including paintings by Rembrandt and Edouard Manet. No one has been arrested.

Authorities have searched Gentile’s home before, including one time with ground penetrating radar in his yard in what Gentile’s lawyer called a veiled attempt to find the artwork. Agents also have removed items from Gentile’s house and searched a backyard shed.

Federal officials declined to say why FBI agents went to Gentile’s home. Gentile's lawyer, A. Ryan McGuigan, who was at the scene, also declined to comment.

“The FBI is conducting court-authorized activity ... in connection with an ongoing federal investigation,” said Kristen Setera, a spokeswoman for the FBI in Boston. “We will have no further comment at this time.”

Gentile has told authorities that he doesn’t know anything about the stolen paintings, and McGuigan has denied prosecutors’ allegations that Gentile is a member of the Philadelphia Mafia.

Assistant U.S. Attorney John Durham has said in court that Gentile once told an undercover FBI agent that he had access to two of the stolen paintings and could negotiate the sale of each for $500,000.

Durham also has said that authorities have evidence Gentile, who currently is charged in a weapons case, talked about the paintings with at least three fellow prisoners at a Rhode Island jail, including giving them information on who to call about the art and what code name to use.

McGuigan has alleged that because the FBI believes Gentile has not been forthcoming with everything he knows about the heist, the agency has set him up for arrests twice in the last three years.

Gentile is charged with selling a loaded .38-caliber revolver for $1,000 to a cooperating government witness at his Manchester home. He has pleaded not guilty and is detained without bail.

In May 2013, Gentile was sentenced to more than two years in prison for illegally selling prescription drugs and possessing guns, silencers and ammunition. In that case, Durham said federal agents found in Gentile’s home a handwritten list of the stolen paintings and their estimated worth, along with a newspaper article about the museum heist a day after it happened.

Three years ago, the FBI in Boston said investigators believed the art thieves belonged to a criminal organization based in New England and the mid-Atlantic. They believe the art was taken to Connecticut and Pennsylvania in the years after the theft and offered for sale in Philadelphia. After that, the trail went cold.

The museum is still offering a $5 million reward for the return of the artwork.