Florence, Colorado - Robert Hanssen, "the most damaging spy" in FBI history, was reportedly found dead in his prison cell Monday.

The 78-year-old was serving a life sentence in ADX prison in Florence, Colorado.

"Staff requested emergency medical services and life-saving efforts continued," Bureau of Prisons (BOP) Director of Communications Kristie Breshears said in a statement first reported by CBS News. "The inmate was subsequently pronounced dead by outside emergency medical personnel."

His cause of death is unclear.

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The 25-year FBI veteran was arrested in Virginia in 2001 after investigators learned he also worked for the Soviet Union and then the Russian Federation.

The counterintelligence expert was apprehended while attempting to deliver classified information to his foreign contacts in a park outside Washington DC.

According to the FBI, that was similar to tasks Hanssen performed successfully many times dating back to 1985, using the alias "Ramon."

The FBI said that by February 2001, roughly 300 agents were assigned to rooting out Hanssen, who was apparently preparing to retire when investigators closed in on him.

He subsidized his FBI income by at least $1.4 million through the arrangements. The disgraced agent pleaded guilty to 15 counts of espionage on July 6, 2001, but not before exploiting his unfettered access to classified information including nuclear secrets and the betrayal of double agents aiding the US.

Hanssen spent several years of his career based in New York City. The married father of six was a Chicago native who studied at Northwestern University. A neighbor in Virginia claimed the spy's wife taught religion classes in Vienna, Virginia.