Mr. Hanssen was sentenced to life in prison in 2002, bringing to a close one of the most lurid and damaging espionage cases in American history.

Robert P. Hanssen, a former F.B.I. agent who spied for Moscow off and on for more than two decades during and after the Cold War in one of the most damaging espionage cases in American history, was discovered dead in his prison cell in Colorado on Monday, federal authorities announced. He was 79.

The Federal Bureau of Prisons said in a statement that Mr. Hanssen was found unresponsive just before 7 a.m. at the United States Penitentiary Florence, where he was serving a life sentence. He was pronounced dead after lifesaving efforts by emergency medical workers. The statement did not identify a cause.

Mr. Hanssen’s case was considered one of the most notorious spy scandals of his generation, shocking F.B.I. leaders and other government officials when they learned that one of their own had been feeding information to the other side with impunity for so many years. To this day, the F.B.I. describes him as “the most damaging spy in bureau history.”

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