One of the most notorious American double agents has died in prison.
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.
He had 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.
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.
He is believed to have died of natural causes, a person familiar with the matter told The Associated Press.
Spy took more than $1.4m in cash and diamonds
Hanssen was paid more than $1.4m (£1.1m) in cash, bank funds, diamonds and Rolex watches in exchange for a wealth of highly classified national security information, including extensive detail about how US officials had tapped into Russian spy operations.
He did not adopt an obviously lavish lifestyle, instead living in a modest suburban home in Virginia with his family of six children.
He would later say he was motivated by money rather than ideology, but in a letter written to his Soviet handlers in 1985 he explained a large payoff could have caused complications because he could not spend it without setting off alarms.
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Under the alias “Ramon Garcia”, Hanssen passed some 6,000 documents and 26 computer discs to his handlers, authorities said.
They detailed eavesdropping techniques, helped confirm the identity of Russian double agents and divulged other state secrets.
Officials also believe he tipped off Moscow to a secret tunnel the Americans had built under the Soviet embassy in Washington for eavesdropping.
Hanssen’s duplicity went undetected for years, but later investigations found several red flags had been missed.
He became the focus of a hunt for a Russian mole and was caught taping a rubbish bag full of secrets to the underside of a footbridge in a park in a “dead drop” for his Russian handlers.
His story was made into a film called Breach in 2007, starring Chris Cooper as Hanssen and Ryan Phillippe as a young bureau operative who helps to bring him down.