Former FBI agent Robert Hanssen was found dead in his prison cell Monday morning while serving a life sentence in federal prison for selling classified material to the Soviet Union. He was 79 years old. According to CBS News, Hanssen was found unresponsive by staff, who initiated life-saving measures. “Staff requested emergency medical services and life-saving efforts continued,” Bureau of Prisons Director of Communications Kristie Breshears said in a statement to CBS News. “The inmate was subsequently pronounced dead by outside emergency medical personnel.” Hanssen began spying for the KGB in 1979—three years after he was hired by the FBI. He used his immense access as an agent to give the Soviets information about American nuclear war preparation, among other classified info. He was arrested in 2001 and pled guilty to 14 counts of espionage and one count to commit espionage. He was subsequently sentenced to 15 life terms.