Robert Hanssen, a former FBI agent who was one of the most damaging spies in American history, was found dead in his prison cell Monday morning, according to the Bureau of Prisons. 

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. 

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.

Three years after he was hired by the FBI, Hanssen approached the Soviets and began spying in 1979 for the KGB and its successor, the SVR. He stopped a few years later after his wife confronted him. 

He resumed spying in 1985, selling thousands of classified documents that compromised human sources and counterintelligence techniques and investigations in exchange for more than $1.4 million in cash, diamonds and foreign bank deposits. Using the alias "Ramon Garcia," he passed information to the spy agencies using encrypted communications and dead drops, without ever meeting in-person with a Russian handler. 

Eric O'Neill, who went undercover for the FBI during its investigation into Hanssen, told CBS News that Hanssen came from a complicated background and had troubles with his father, who wanted him to go into medicine. But Hanssen, who did go to dentistry school, wanted to be in law enforcement. 

"He really wanted to catch spies. He was a James Bond fanatic, loved the movies," O'Neill said. "He could quote them chapter and verse. He wanted to be a spy. He was joining the FBI to do that — not to spy against the U.S., but to go in and hunt spies." 

But he was angry when he didn't get the exact job he wanted at the FBI, and taking care of his growing family while living in New York and later the Washington, D.C., area was expensive. 

"And that led him to decide that he was going to get everything he wanted — become a spy," O'Neill said. 

His job in the FBI gave him unfettered access to classified information on the bureau's counterintelligence operations. His disclosures included details on U.S. nuclear war preparations and a secret eavesdropping tunnel under the Soviet embassy in Washington, D.C. He also betrayed double agents, including Soviet Gen. Dmitri Polyakov, who were later executed. 

Hanssen was arrested after making a dead drop in a Virginia park in 2001 after the FBI had been secretly monitoring him for months. His identity was discovered after a Russian intelligence officer handed over a file containing a trash bag with Hanssen's fingerprints and a tape recording of his voice. 

In letters to the KGB, Hanssen expressed concern that he might one day be caught, and he often checked FBI computers for any sign that it was investigating him. 

"Eventually I would appreciate an escape plan. (Nothing lasts forever.)," he wrote in 1986, according to the FBI affidavit. 

Hanssen never revealed his motivation for spying. But O'Neill, who wrote a book about the investigation to nab Hanssen, has some theories. 

"He truly didn't respect Russia very much, at least not in his conversations with me," O'Neill said. "But he was able to use them very effectively to solve his other problems. One that he was angry at the FBI for not placing him in the position of authority and gravitas and respect that he believed he deserved. And two, he needed money. He was financially having problems and he needed money and you solve both those problems by becoming a spy." 

"At some point, spying and being the top spy for the Soviet Union, while within the FBI, became the thing that made him belong to something much bigger than himself," he added. "I think that at some point, even more than the money that became what was so important to him." 

Hanssen's life in prison was "absolutely horrible," O'Neill said. He spent 23 hours a day alone in a tiny cell. 

In:
• Spying
• Russia
• Obituary

Caitlin Yilek

Caitlin Yilek is a politics reporter for CBS News Digital. Reach her at [email protected]. Follow her on Twitter: https://twitter.com/hausofcait

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