WASHINGTON (Reuters) -&nbsp;Robert&nbsp;Hanssen, the former&nbsp;FBI&nbsp;agent&nbsp;turned spy&nbsp;whom the bureau describes as the most damaging in its history, was&nbsp;found&nbsp;dead&nbsp;in his&nbsp;prison&nbsp;cell on Monday, U.S. authorities said.
Hanssen, 79, was sentenced in 2002 to life in&nbsp;prison&nbsp;after pleading guilty to spying for the Soviet Union and later&nbsp;Russia&nbsp;for over 20 years.
Prison&nbsp;staff initiated life-saving measures after finding&nbsp;Hanssen&nbsp;unresponsive on Monday morning but were not successful, the Bureau of&nbsp;Prisons said in a statement. It did not provide a cause of death.
Hanssen&nbsp;joined the&nbsp;FBI&nbsp;in 1976 and began selling classified information to the Soviet Union in 1985, according to the&nbsp;FBI's website.
By the time of his arrest in 2001, he had been compensated with more than $1.4 million in cash, bank funds and diamonds, in exchange for compromising numerous human sources, intelligence techniques and classified U.S. documents, the&nbsp;FBI's website says.
FBI&nbsp;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&nbsp;Hanssen, according to the&nbsp;FBI.
An arrest team took&nbsp;Hanssen&nbsp;into custody after catching him making a "dead&nbsp;drop" of classified materials in a park in suburban Virginia, the&nbsp;FBI&nbsp;says.
He had been serving his life sentence in a maximum security facility in Colorado.
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(Reporting by Rami Ayyub, Editing by Rosalba O'Brien)

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Robert Hanssen, FBI agent who spied for Russia, found dead in prison