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.

Robert Hanssen was discovered at a maximum-security facility in Florence, Colorado, on Monday morning.

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Hanssen was found unresponsive and staff immediately initiated life-saving measures, Bureau of Prisons spokeswoman Kristie Breshears said in a statement. 

“Staff requested emergency medical services and life-saving efforts continued,” Breshears said. “The inmate was subsequently pronounced dead by outside emergency medical personnel.” 

Hanssen appears to have died of natural causes, according to two sources briefed on the matter.

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Hanssen, 79, was arrested in 2001 and pleaded guilty to selling highly classified material to the Soviet Union and later Russia. He was serving a life sentence at the federal penitentiary in Florence, Colorado. 

In 1979, Hanssen approached the Soviets three years after being hired by the FBI, and he started spying for the KGB and its successor, the SVR. A few years later, when his wife confronted him, he stopped.

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In 1985, he picked up espionage again, selling tens of thousands of top-secret documents that jeopardized counterintelligence methods and investigations for more than $1.4 million in cash, jewels, and foreign bank deposits. He never really interacted with a Russian handler while operating under the alias “Ramon Garcia,” instead providing intelligence to the espionage services via dead drops and encrypted communications.

While there was some suspicion around his unusual activities occasionally, he was not caught for years.

After the spy Aldrich Hazen Ames was arrested by the FBI in 1994, the bureau realised classified information was still being leaked, which is what instigated the investigation into Hanssen.

He had been due to retire so the FBI acted quickly in an effort to catch him “red handed”.