Robert Hanssen was sentenced to life in prison in 2002 after he pleaded guilty to 15 counts of espionage and conspiracy (Representational image)

Robert Hanssen, a former FBI agent notorious as one of the most damaging spies in US history, was found dead in his cell in Colorado yesterday. He was 79.

Hanssen was accused of being a Russian mole and spying for Moscow during and after the Cold War for more than two decades in exchange for more than $1.4 million in cash and diamonds.

He was sentenced to life in prison in 2002 after he pleaded guilty to 15 counts of espionage and conspiracy. Even to this day, the FBI describes him as “the most damaging spy in bureau history.”

 Hanssen began spying in 1979, just three years after he joined the FBI.