Robert Hanssen was an American former FBI agent who was arrested in 2001 for espionage. He was convicted of spying for the Soviet Union and later for Russia over a period of more than 20 years, compromising numerous intelligence operations and sources. His actions caused significant damage to U.S. national security. Hanssen's espionage activities included passing classified information, which he often delivered in exchange for money. He was sentenced to life in prison without the possibility of parole. His case is considered one of the most damaging spy cases in U.S. history, highlighting vulnerabilities within the FBI.