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Robert Hanssen, the former FBI agent turned spy whom the bureau describes as the most damaging in its history, was found dead in his prison cell on Monday, U.S. authorities said.
Hanssen, 79, was sentenced in 2002 to life in prison after pleading guilty to spying for the Soviet Union and later Russia for over 20 years.
Prison staff initiated life-saving measures after finding Hanssen unresponsive on Monday morning but were not successful, the Bureau of Prisons said in a statement.
