NEW YORK

Former FBI agent Robert Hanssen, who was convicted of spying for Russia, has died at age 79, the Federal Bureau of Investigation said on Monday.

The FBI said that Hanssen became the most ''damaging spy'' in bureau history despite taking an oath to enforce the law and protect the nation.

Hanssen was found dead in his prison cell, where he was serving a life sentence, according to the Bureau of Prisons.

He was arrested in 2001 and accused of providing highly classified national security information to the Russians in exchange for more than $1.4 million in cash, bank funds, and diamonds.

Hanssen’s espionage activities began in 1985, the FBI said, and in 2001 he pled guilty to 15 counts of espionage.