By Richard Weizel

MILFORD, Conn. (Reuters) - Police in Connecticut on Friday arrested a 78-year-old man with a long criminal history on a gun charge, but the man's lawyer claimed the arrest was a ruse intended to pressure him to talk about the biggest art heist in U.S. history.

Robert Gentile appeared in federal court in Hartford, Connecticut, on Friday and was charged with selling a firearm to an undercover agent, a crime that could carry a 10-year prison charge if he is convicted due to his status as a felon.

His attorney, Ryan McGuigan, contended Federal Bureau of Investigation agents really want to see what he knows about the theft of $500 million in artwork from Boston's Isabel Stuart Gardner Museum 25 years ago.

"The FBI has been questioning him for years because they think he knows the whereabouts of the Gardner paintings," McGuigan said. "But he's being set up, just like others who are threatened with long periods of incarceration by the FBI in the Gardner case. The gun charge is just an excuse to question and threaten him further."

Gentile denies any knowledge of the heist, McGuigan said.

The FBI and U.S. Attorney's Office in Connecticut declined to comment on McGuigan's claim.

The Gardner heist took place on the rainy night of March 18, 1990, when two men posing as police officers arrived at the museum's front door. The guards on duty let them in and the pair allegedly overpowered them.

The next morning the guards were found duct-taped to chairs in the basement and 13 artworks including Rembrandt's "Storm on the Sea of Galilee" and Vermeer's "The Concert" were missing. Due to a quirk in the museum founder's will, the empty frames that once held those paintings remain on the walls, a ever-present reminder of one of Boston longest-unsolved crimes.

FBI investigators have long said they believed organized crime groups were behind the heist.

Gentile in 2012 submitted to a lie detector test that showed a high likelihood that he had lied when he investigators asked if he had any knowledge of the heist, according to court testimony.

A 2012 FBI search of Gentile's home turned up a list of the stolen art pieces and their estimated value, as well as police uniforms.

Gentile has a criminal record dating back to the 1950s. Most recently he spent two years in prison after being convicted of illegally selling prescription drugs to an informant.

(Editing by Scott Malone; Editing by Sandra Maler)