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A watch tower stands behind barbed wire fences.
A guard tower looms over a federal prison complex which houses a Supermax facility outside Florence in this 2015 file photo. (Brennan Linsley, AP Photo, File)

Robert Hanssen, a former FBI agent who admitted spying for the Soviet Union and Russia in exchange for $1.4 million in cash and diamonds, died Monday morning in a federal prison in southern Colorado, officials said.

Hanssen, 79, was found unresponsive about 6:55 a.m. at the United State Penitentiary Florence ADMAX in Florence, according to a news release from the Federal Bureau of Prisons. Prison staff initiated life-saving measures and called EMS, before he was pronounced dead.

He is believed to have died of natural causes, The Associated Press reported. No official cause of death was released. 

Hanssen, whom the FBI called the “most damaging spy” in its history, was arrested in 2001 and later pleaded guilty to 15 counts of espionage for selling highly classified national security information to Russians. He was sentenced to life in prison without parole. 

He had been serving his sentence in Florence since July 17, 2002.

Olivia Prentzel is a general assignment writer based in Colorado Springs for The Colorado Sun, covering breaking news, wildfires and all things interesting impacting Coloradans. Before joining The Sun, Olivia covered criminal justice for The Colorado Springs Gazette. She’s also worked at newspapers in New Orleans and New Jersey, where she grew up. After graduating college, she lived in a tiny, rural town in southern Madagascar for three years as a Peace Corps volunteer. When not writing, Olivia enjoys backpacking and climbing Colorado’s tallest peaks.