Robert Philip Hanssen (April 18, 1944 – June 5, 2023) was an American Federal Bureau of Investigation (FBI) agent who spied for Soviet and Russian intelligence services against the United States from 1979 to 2001. His espionage was described by the Department of Justice as "possibly the worst intelligence disaster in U.S. history". In 1979, three years after joining the FBI, Hanssen approached the Soviet Main Intelligence Directorate (GRU) to offer his services, beginning his first espionage cycle, lasting until 1981. He restarted his espionage activities in 1985 and continued until 1991, when he ended communications during the collapse of the Soviet Union, fearing he would be exposed. Hanssen restarted communications the next year and continued until his arrest. Throughout his spying, he remained anonymous to the Russians. Hanssen sold thousands of classified documents to the KGB that detailed U.S. strategies in the event of nuclear war, developments in military weapons technologies, and aspects of the U.S. counterintelligence program. He was spying at the same time as Aldrich Ames in the Central Intelligence Agency (CIA). Both Ames and Hanssen compromised the names of KGB agents working secretly for the U.S., some of whom were executed for their betrayal. Hanssen also revealed a multimillion-dollar eavesdropping tunnel built by the FBI under the Soviet Embassy. After Ames' arrest in 1994, some of these intelligence breaches remained unsolved, and the search for another spy continued. The FBI paid 1.4 million in cash and diamonds over twenty-two years.