Robert Hanssen, a former FBI agent who was convicted of spying for the Soviet Union and Russia, has been found dead in his prison cell. He was serving a life sentence at the ADX Florence supermax prison in Colorado.

Hanssen, who was 79 years old, had been in custody since his arrest in 2001. He admitted to selling classified information to the Russians for over 20 years, including details on the US nuclear arsenal and intelligence-gathering methods.

His crimes were discovered when the FBI learned that a Russian spy had been communicating with an agent in the bureau’s New York field office. Hanssen eventually confessed to his role as the spy, and agreed to cooperate with US authorities in exchange for a promise that his wife would receive a financial settlement.

Hanssen was sentenced to life in prison without the possibility of parole in 2002. He had been held in solitary confinement at ADX Florence, which is known for its strict security measures and high-profile inmates.

The cause of Hanssen’s death has not been released, and the FBI has not commented on the matter. However, his attorney has confirmed his passing and offered condolences to his family.

Hanssen’s case remains one of the most notorious examples of espionage in US history, and his actions are said to have caused significant damage to national security.