BOSTON — Federal agents trying to solve the biggest art theft in the nation’s history arrested a 79-year-old Hartford man on Friday after conducting their second sting against him in three years to force him to disclose the whereabouts of $500 million in stolen art, the man’s lawyer said.

The suspect, Robert V. Gentile, was arrested by F.B.I. agents on charges of selling a .38 Colt cobra revolver on March 2 to an unidentified man who was acting as a confidential informant for the authorities. Investigators say Mr. Gentile, who has been on probation as a result of a 2013 conviction that was part of the first Federal Bureau of Investigation sting against him, received $1,000 for the sale.

Mr. Gentile’s lawyer, A. Ryan McGuigan, said his client knew nothing about the art theft, which occurred 25 years ago at the Isabella Stewart Gardner Museum in Boston and included works by Rembrandt and Vermeer. Officials investigating the robbery have said in recent weeks that they thought Mr. Gentile remained their strongest lead in the frustrating case because most of the men they had identified as suspects in the robbery were dead.

“It’s the same F.B.I. guys doing the same thing as last time,” Mr. McGuigan said. “They won’t stop squeezing my client.”