Robert Philip Hanssen took the same oath as every other FBI agent, swearing to 'enforce the law and protect the nation'. And yet, he ended up being the most corrupted spy in US history.
After a few years of working as a spy for the FBI, Hanssen decided to commit espionage and reveal top secret information to Moscow for 30 years in what has been dubbed 'the worst spy disaster to ever happen.'
Hanssen used the alias 'Ramon Garcia' when working with Russia, and provided them with highly classified national security information in exchange for more than $1.4 million in cash, bank funds, and diamonds.
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Throughout this time, the FBI knew that there was a mole who was passing on information to Russia's KGB, and put their best men on the case, with one of them being Hanssen himself.
The guy was essentially chasing himself down, and his colleagues were none the wiser.
Even though Hanssen had done a good job at keeping under the radar and delivering classified information for six whole years (until he ended his communication with Russia in 1991 following the collapse of the Soviet Union), he feared he would be exposed.
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That was, however, until December 2000, FBI agent Richard Garcia had a curious visit from a colleague overseeing the Russia desk.
"He asked, 'Do you know a guy named Robert Hanssen?'" Mr Garcia recalled. "I said, 'No'."
The officer then told him: "Good. Because you're about to."
The two agents then took on the case together as a partnership, looking into Hanssen's activities, which they came to find a 100-page affidavit outlining his crimes, Hanssen's espionage resulted in the arrest and imprisonment of three US sources and the execution of two more.
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After suspicions became overwhelming, the FBI paid $7 million to a KGB agent to obtain a file on an anonymous mole, which was soon identified to be Hanssen through fingerprint analysis.
He later admitted that he was 'motivated by money' rather than ideology, but a letter written to his Soviet handlers in 1985 explains that a large payoff could have caused 'complications' because he could not spend it without setting off warning bells.
Hanssen was then charged and pled guilty to 15 counts of espionage on 6 July, 2001, and was sentenced to life in prison on 10 May, 2002.
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On Monday, (12 February) it was confirmed by prison officials that Hanssen, 79, was found unresponsive in his cell at a federal prison in Florence, Colorado - and later pronounced dead. He is believed to have passed by natural causes.