Robert Hanssen, the former FBI agent turned spy who was described by the bureau as the most damaging in its history, was found dead in his prison cell on Monday, US authorities said.
Hanssen, 79, was sentenced in 2002 to life in prison after pleading guilty to spying for the Soviet Union and later Russia for more than 20 years.
Prison staff tried to resuscitate Hanssen after finding him unresponsive on Monday morning but were not successful, the Bureau of Prisons said. It did not provide a cause of death.
Hanssen joined the FBI in 1976 and began selling classified information to the Soviet Union in 1985, according to the FBI's website.
By the time of his arrest in 2001, he had been compensated with more than $1.4 million in cash, bank funds and diamonds, in exchange for compromising human sources, intelligence techniques and classified US documents, it says.
FBI investigators worked for years to try to identify the spy in their ranks.
In the weeks leading up to his February 2001 arrest, about 300 personnel were working on the investigation and monitoring Hanssen, the FBI says.
An arrest team took him into custody after catching him making a "dead drop" of classified material in a park in suburban Virginia, the FBI says.
Hanssen had been serving his life sentence in a maximum-security prison in Colorado.
