Robert Hanssen, a former federal agent who pleaded guilty two decades ago to spying for the Soviet Union, died Monday morning in the Supermax prison in southern Colorado. He was 79.

Prison staff at the United States Penitentiary Florence ADMAX in Florence found Hanssen unresponsive around 6:55 a.m., the Bureau of Prisons said in a news release. They attempted life-saving measures before pronouncing him dead, the release stated.

No staff or other inmates were injured, prison officials said.

Hanssen had been serving time in the prison known as the “Alcatraz of the Rockies” since July 17, 2002. He was arrested in 2001 and pleaded guilty to selling highly classified material to the Soviet Union and, later, Russia.