The 79-year-old former Vienna, VA resident was apparently found unresponsive just before 7 a.m. on Monday, June 5, the Washington Post reports. Purported attempts to save him were unsuccessful.
Hanssen's case is considered among the most egregious espionage cases in US history.
In February 2001, federal investigators learned Hanssen was set to make a dead drop at Foxstone Park in Fairfax County, a place where he'd ben spotted before by FBI surveillance, according to the FBI.
"Hanssen parked on a residential street and walked down a wooded path to a footbridge with the classified materials wrapped in a plastic bag," the FBI writes. "As Hanssen walked back to his car, the arrest team rushed up and took him into custody."
An investigation found Hanssen had handed over thousands of highly-classified documents detailing U.S. strategies in the event of nuclear war, major developments in military weapons technologies, information on active espionage cases, and many other aspects of the U.S. Intelligence Community's Soviet counterintelligence program to the KGB, the USDOJ said.
Hanssen pleaded guilty to 15 counts of espionage for spying for Moscow, both during and after the Cold War, and was sentenced to life in prison in 2002.
A cause of death was not immediately disclosed.
Click here for more on Hanssen's case from the FBI.
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