Robert Hanssen, the spy turned former FBI agent that the agency referred to as the most damaging in its history, was found dead in his prison cell on Monday, according to the news agency AP.
After pleading guilty to spying for Russia and the Soviet Union for more than two decades, Hanssen, then 79, received a life sentence in 2002.
Jail staff started life-saving measures after tracking down Hanssen, who was unresponsive on Monday morning, but they were not successful, the Bureau of Prisons said in a statement.
He is believed to have died of natural causes, an individual acquainted with the matter told The Associated Press.
The individual spoke to the Associated Press on condition of anonymity because they were not permitted to discuss the specifics of Hanssen's death in public.
After pleading guilty to 15 counts of espionage and other charges, he had been sentenced to life in prison without the possibility of parole since 2002.
Hanssen had uncovered a wealth of data about American intelligence-gathering, including broad insight concerning how U.S. authorities had taken advantage of Russian spy operations, since 1985.
At least three Soviet officers who were working for U.S. intelligence and who were executed after being exposed were thought to have been partially to blame.
In exchange for providing the Soviet Union and, later, Russia with highly classified information regarding national security, he received more than $1.4 million in cash, bank funds, diamonds, and Rolex watches.
He didn't take on a clearly sumptuous way of life, rather residing in a humble rural home in Virginia with his group of six children and driving a Taurus and minivan.
Hanssen would later claim that he was motivated by money rather than ideology; however, a 1985 letter to his Soviet handlers explains that a substantial payoff could have caused complications because he was unable to spend it without setting off warning bells.
Utilizing the false name "Ramon Garcia," he passed around 6,000 archives and 26 computer disks to his overseers, authorities said.