WASHINGTON — Robert Hanssen, a former FBI agent who took more than $1.4 million in cash and diamonds to trade secrets with Moscow in one of the most notorious spying cases in American history, died in prison Monday.

Hanssen, 79, was found unresponsive in his cell at a federal prison in Florence, Colorado, and later pronounced dead, prison officials said. He 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 possibility of parole since 2002, after pleading guilty to 15 counts of espionage and other charges.

Hanssen had divulged a wealth of information about American intelligence-gathering, including extensive detail about how U.S. officials had tapped into Russian spy operations, since at least 1985.

He was believed to have been partly responsible for the deaths of at least three Soviet officers who were working for U.S. intelligence and executed after being exposed.

He got more than $1.4 million in cash, bank funds, diamonds and Rolex watches in exchange for providing highly classified national security information to the Soviet Union and later Russia.

He didn't adopt an obviously lavish lifestyle, instead living in a modest suburban home in Virginia with his family of six children and driving a Taurus and minivan.

Hanssen would later say he was motivated by money rather than ideology, but a letter written to his Soviet handlers in 1985 explains a large payoff could have caused complications because he could not spend it without setting off warning bells.

Using the alias "Ramon Garcia," he passed some 6,000 documents and 26 computer disks to his handlers, authorities said. They detailed eavesdropping techniques, helped to confirm the identity of Russian double agents, and spilled other secrets. Officials also believed he tipped off Moscow to a secret tunnel the Americans built under the Soviet Embassy in Washington for eavesdropping.

He went undetected for years, but later investigations found missed red flags. After he became the focus of a hunt for a Russian mole, Hanssen was caught taping a garbage bag full of secrets to the underside of a footbridge in a park in a "dead drop" for Russian handlers.

The FBI has been notified of Hanssen's death, according to the Bureau of Prisons.