Former FBI special agent Robert Hanssen, who was convicted of spying for Russia, was found dead at the federal supermax prison in Colorado, the Federal Bureau of Prisons announced.

Hanssen, 79, was found unresponsive on Monday at 6:55 a.m., the FBOP confirmed to KLTA sister station KDVR. Staff at the Florence prison, officially known as USP ADX Florence, initiated life-saving measures while requesting assistance from emergency personnel, but Hanssen was later pronounced dead.

No staff or other inmates were injured, according to FBOP.

According to the FBI, Hanssen became a special agent in 1976 and would go on to be known by the agency as “the most damaging spy in Bureau history.”

On Feb. 18, 2001, Hanssen was arrested and charged with committing espionage on behalf of Russia and the former Soviet Union. The FBI said Hanssen had provided highly classified national security information to the Russians in exchange for more than $1.4 million in cash, bank funds and diamonds.

Hanssen pleaded guilty to multiple charges and was sentenced to 15 life sentences. He had been at ADX Florence since July 17, 2002.

ADX is an administrative maximum security penitentiary located in Colorado and is home to some of the most infamous criminals. ADX has been coined the “Alcatraz of the Rockies.”

Hanssen’s cause of death had not been released as of publishing time Monday.