It remains the largest art heist in history, a brazen robbery in which two thieves disguised as police officers walked into Boston’s Isabella Stewart Gardner Museum, disabled two guards and stole masterworks worth more than half a billion dollars.

The 1990 robbery and the recovery of the paintings have puzzled investigators for more than two decades.

Now federal authorities appear to be pinning some hope of solving the mystery on Robert Gentile, a 75-year-old reputed mobster who is jailed in a drug case.

The FBI believes Gentile “had some involvement in connection with stolen property” related to the art heist, said assistant U.S. attorney John Durham in Hartford federal court. Durham said FBI agents have had unproductive discussions with Gentile about the theft, but didn’t elaborate on his allegations.

Gentile’s attorney, A. Ryan McGuigan, called the notion preposterous. He said Gentile has lived with his wife in the same small house in a Hartford suburb for 50 years and has no idea what prosecutors are talking about.

“He doesn’t know anything about art, he’s never been to an art gallery in his life,” McGuigan said in an interview. He “couldn’t tell a Rembrandt from an Elvis painting.”