Robert Hanssen, a former F.B.I. agent who was sentenced to life in prison in 2002 for spying for Moscow during and after the Cold War in one of the most damaging espionage cases in American history, was found dead in his cell in a federal prison in Colorado on Monday, the Federal Bureau of Prisons said. He was 79.

The bureau said in a statement that Mr. Hassen was found unresponsive at the United States Penitentiary Florence in Colorado just before 7 a.m. He was pronounced dead after lifesaving efforts by emergency medical workers.

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