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. Hanssen 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.

The statement did not give a cause for Mr. Hanssen’s death.

Mr. Hanssen had been in custody at the Florence lockup, a maximum-security facility, since July 17, 2002, after he was sentenced to life in prison without parole in May of that year after pleading guilty to 15 counts of espionage.

Mr. Hanssen joined the F.B.I. in 1976 as a special agent and went on to hold several counterintelligence positions that gave him access to classified information. He began spying for the Soviet Union three years after joining the bureau, when he was assigned to a counterintelligence unit in New York, by walking into the New York offices of Amtorg, a Soviet trade organization that was known to be a front for the Soviet military intelligence agency.

He stopped spying for several years starting in 1980, after his wife, Bonnie, walked in on him in the basement of their home in Westchester County, N.Y., and he quickly tried to cover up his papers. He confessed to her and to a priest affiliated with Opus Dei, the conservative Catholic organization to which the couple belonged.

In 1985, he began spying again, providing information to the K.G.B. This time, he did a better job of covering his tracks, using encrypted communications and other secret methods. Identifying himself only by code names like B and Ramon Garcia, Mr. Hanssen told the K.G.B. about three of its officers who were spying for the United States and also revealed the existence of a tunnel that the F.B.I. and the National Security Agency had built beneath the Soviet Embassy in Washington. Government officials said his betrayal of the tunnel cost the United States hundreds of millions of dollars.

Mr. Hanssen’s work as a spy for Moscow went undetected for years as he collected more than $1.4 million in cash and diamonds in exchange for “highly classified national security information,” according to the F.B.I.

In the 1990s, after the arrest of Aldrich Ames, a C.I.A. agent who also spied for the Russians, the F.B.I. and the C.I.A. realized that someone was still providing Russia with classified information. But it wasn’t until 2000 that the agencies were able to find Russian documentation that led them to Mr. Hanssen.

He was arrested in a Virginia park in February 2001 after he left classified documents for his Russian handlers.

This is a developing story.