An aging former convict who has been linked to the masterpieces stolen from the Isabella Stewart Gardner Museum is seeking the dismissal of a weapons indictment against him, arguing that authorities improperly brought the case in an effort to force him to cooperate in the investigation into the historic art heist.
A lawyer for Robert Gentile, 79, filed a motion to dismiss the indictment Tuesday in federal court in Connecticut, where Gentile is charged with selling a gun to a felon who was working as an FBI informant.
“It is Mr. Gentile’s position that the instant firearms prosecution was . . . engineered solely for the purpose of coercing cooperation into the art heist investigation, rather than removing an armorer-to-felons from our midst,” attorney A. Ryan McGuigan, wrote in the motion.
A spokesman for US Attorney Deirdre M. Daly declined to comment Thursday.
McGuigan wrote that just before Gentile’s arrest in April for the alleged gun sale, FBI agents and a prosecutor informed him of the charges he would face and urged him to assist with the Gardner investigation. Gentile denied knowing where the paintings are.
The current case is the second sting operation that has ensnared Gentile, who was arrested in 2012 for selling prescription drugs to another informant. He pleaded guilty in the same court and received a 30-month prison term. McGuigan asserted then that the government brought the drug case in an effort to compel Gentile’s cooperation in the Gardner probe.
While McGuigan has repeatedly denied that Gentile knows where the paintings are, prosecutors have said that he told an undercover FBI agent that he could sell two of them.
McGuigan wrote Tuesday that Gentile cooperated with authorities in 2010 and 2011, having recorded conversations with people linked to the theft. But that cooperation ended when Gentile, fearing for his family’s safety, balked at bringing an informant to a meeting with “violent criminal associates” of a potential suspect in the heist, McGuigan wrote.
The theft occurred on March 18, 1990. Two men dressed as police officers talked their way into the Boston museum, tied up two guards, and fled with $500 million worth of masterpieces.
Gentile pleaded not guilty in the gun case and is being held pending trial in October.
McGuigan wrote Tuesday that if Gentile is freed, “he can expect an acquaintance turning up on his doorstep . . . proposing a criminal joint venture, with the same cast of FBI agents hovering at a discreet distance.”
Shelley Murphy of the Globe Staff contributed to this report.