Robert Hanssen, one of the most notorious spies ever convicted in the U.S., died in a federal prison in Colorado on Monday. He was 79 years old.
Prison officials said Hanssen was found unresponsive in his cell at a prison in Florence, Colorado, and later pronounced dead. A person with knowledge of the incident anonymously told the Associated Press that Hanssen appeared to have died of natural causes.
Hanssen worked as an FBI special agent starting in 1976. Starting in 1985, he used his access to send classified U.S. national security information to the Soviet Union, and later to Russia's Foreign Intelligence Service. He used methods like dead drops to pass along information, and in return received cash and gemstones worth about $1.4 million total.
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The 21-year-old was arrested in April after a massive trove of military records were discovered on the social messaging platform Discord.
When it suspected Hanssen was spying, the FBI arranged a bugged office from him to work from, and ultimately arrested him in 2001 when he was returning from leaving classified materials at a dead drop in a park in Virginia.
According to the FBI, Hanssen's espionage compromised U.S. counterintelligence operations, human sources, and dozens of classified documents.
In 2001 he pleaded guilty to 15 counts of espionage and other crimes. He had been at the prison since July of 2002, where he was serving a life sentence without parole.
The Bureau of Prisons says it has notified the FBI of Hanssen's death.
