Robert Hanssen, a former FBI agent turned spy whom the bureau describes as the most damaging in its history, was found dead in his prison cell, United States authorities have 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 initiated life-saving measures after finding Hanssen unresponsive on Monday morning but were not successful, the Bureau of Prisons said in a statement. It did not provide a cause of death.
Hanssen, who was serving his sentence in Colorado, joined the Federal Bureau of Investigation in 1976 and began selling classified information to the Soviet Union in 1985, according to the FBI.
By the time of his arrest in 2001, he had been compensated with more than $1.4m in cash, bank funds and diamonds, in exchange for compromising numerous human sources, intelligence techniques and classified US documents, the bureau says on its website.
“The FBI trusted him with some of the most sensitive secrets of the US government, and instead of upholding that trust, he abused and betrayed it,” the FBI says, adding that Hanssen used his “experience and training as a counterintelligence agent” to go undetected for years.
Former FBI Director Louis Freeh described Hanssen’s conduct at the time of his arrest as the “most traitorous action imaginable” against the US.
FBI investigators worked for years to try to identify the spy in their ranks.
In the weeks leading up to Hanssen’s February 2001 arrest, some 300 personnel were working on the investigation and monitoring him, according to the FBI.
An arrest team took Hanssen into custody after catching him making a “dead drop” of classified materials in a park in suburban Virginia, the FBI says.
The US government decided not to seek the death penalty against Hanssen after he entered a plea agreement that saw him cooperate with US intelligence agencies. (AlJazeera)