Robert Hanssen, the former FBI agent who betrayed his country by selling secrets to the Soviet Union and Russia for more than two decades, has died in a top-security prison in Colorado.

He was 77 years old. Hanssen was sentenced to life imprisonment without parole in 2002 after pleading guilty to 15 counts of espionage and conspiracy.

He was considered one of the most damaging spies in U.S. history, compromising dozens of agents and operations and exposing vital intelligence sources and methods.

He was also accused of passing information that led to the execution of at least three Soviet agents who had cooperated with the U.S. Hanssen’s motives for spying were never fully explained, but he reportedly received more than $1.4 million in cash and diamonds from his handlers.

He also claimed to have acted out of boredom and resentment towards his superiors. Hanssen spent his last years in solitary confinement at the ADX Florence prison, also known as the “Supermax”, where he had no contact with other inmates or visitors.

He died of natural causes on Monday, according to prison officials.