WASHINGTON, June 5 (Reuters) - Robert Hanssen, the former FBI agent turned spy whom the bureau describes as the most damaging in its history, was found dead in his prison cell on Monday (Jun 5), US authorities said.

Hanssen, 79, was sentenced in 2002 to life in prison after pleading guilty to spying for the Soviet Union and later Russia for over 20 years.

Prison staff initiated life-saving measures after finding Hanssen unresponsive on Monday morning but were not successful, the Bureau of Prisons said in a statement. It did not provide a cause of death.

Hanssen joined the FBI in 1976 and began selling classified information to the Soviet Union in 1985, according to the FBI's website.

By the time of his arrest in 2001, he had been compensated with more than US$1.4 million in cash, bank funds and diamonds, in exchange for compromising numerous human sources, intelligence techniques and classified US documents, the FBI's website says.

FBI investigators worked for years to try to identify the spy in their ranks. In the weeks leading up to his February 2001 arrest, some 300 personnel were working on the investigation and monitoring Hanssen, according to the FBI.

An arrest team took Hanssen into custody after catching him making a "dead drop" of classified materials in a park in suburban Virginia, the FBI says.

He had been serving his life sentence in a maximum security facility in Colorado.