A former FBI agent who took more than $1.4 million in cash and diamonds to trade secrets with Russia and the former Soviet Union in one of the most notorious spying cases in American history died in prison Monday.

Robert Hanssen, 79, was found unresponsive in his cell at a federal prison in Florence, Colorado, and later pronounced dead, prison officials said.

Hanssen is believed to have died of natural causes, a person familiar with the matter told The Associated Press. The person was not authorized to publicly discuss details of Hanssen’s death and spoke to the AP on condition of anonymity.

He had been serving a sentence of life in prison without parole after pleading guilty to 15 counts of espionage and other charges in 2001.

He had begun providing highly classified national security information to Moscow in 1985, and got more than $1.4 million worth of cash, bank funds and diamonds in exchange over the course of 16 years, according to an FBI history of his case.

The FBI has been notified of Hanssen’s death, according to the Bureau of Prisons. He had been serving time in Colorado since July 2002.

(AP)