Robert Hanssen divulged information about US intelligence-gathering since at least 1985 and was believed to have been partly responsible for the deaths of at least three Soviet officers who were working for US intelligence and were executed after being exposed.
Robert Hanssen, a former FBI agent, was sentenced in 2002 to life behind bars after pleading guilty to spying for the Soviet Union and later Russia for over 20 years.
The 79-year-old was found unresponsive in his cell at a federal prison in Florence, Colorado, on Monday and was later pronounced dead, prison officials said.
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