(CBS NEWS) -- Robert Hanssen, a former FBI agent who was one of the most damaging spies in American history, was found dead in his prison cell Monday morning, according to the Bureau of Prisons.
Hanssen, 79, was arrested in 2001 and pleaded guilty to selling highly classified material to the Soviet Union and later Russia. He was serving a life sentence at the federal penitentiary in Florence, Colorado.
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Hanssen was found unresponsive and staff immediately initiated life-saving measures, Bureau of Prisons spokeswoman Kristie Breshears said in a statement.