One of the most damaging double agents in modern American history, Robert Hanssen gave the Soviets, and later the Russians, thousands of pages of classified material that revealed such sensitive national security secrets as the identities of Soviets spying for the U.S., specifics about America’s nuclear operations and the existence of an FBI-built tunnel underneath the Soviet Embassy in Washington.

Hanssen’s double life began in 1979 and ended in 2001, when he was arrested after the FBI discovered, thanks to help from an ex-KGB officer, that Hanssen was a mole. A church-going father of six, Hanssen was thought to have been motivated by money rather than ideological beliefs. While covertly working for Moscow on and off over the years, he was paid $600,000 in cash and diamonds, with another $800,000 supposedly held for him in a Russian bank. Hanssen was only the third agent in FBI history charged with spying.

Born in 1944, Hanssen was a Chicago native and son of a police officer. He graduated from Knox College in 1966 then attended dental school at Northwestern University before quitting the program to earn an MBA. He went on to work as an investigator for the Chicago Police Department then joined the FBI in 1976. He worked for the agency in Indiana and later New York City.

Hanssen’s deceit began in 1979, when he volunteered to spy for GRU, the Soviet military intelligence agency. He soon informed the Soviets that one of their generals, Dmitri Polyakov, was in fact a CIA informant who’d been spying for America since the 1960s. The Soviets eventually executed Polyakov.

In 1980, after Hanssen’s wife reportedly caught him with some suspicious-looking papers, he admitted to selling secrets to the Soviets, but claimed the information he’d given them was worthless. At his wife’s insistence, Hanssen promised to sever ties with the Soviets and confessed to a priest, who told him to donate the dirty money to charity. However, in 1985, Hanssen resumed his espionage activities, this time for the KGB. He gave the KGB the names of three Soviet officers collaborating with the CIA and FBI. The three spies were arrested and executed.

Meanwhile, Hanssen continued to rise through the FBI’s ranks, eventually working in senior counterintelligence roles. In 1991, with the Soviet Union breaking apart, he stopped spying, possibly due to fears that he’d be found out. But In 1999, while serving as the FBI liaison to the U.S. State Department, he once again resumed his double-agent career, this time for the SVR, a post-Soviet, Russian intelligence service.

Hanssen’s downfall came in 2000 when the FBI, which by then suspected there was a mole in its ranks, paid $7 million to an ex-KGB officer to procure information from SVR headquarters that helped identify Hanssen as the turncoat. The FBI put Hanssen under surveillance in late 2000, and on February 18, 2001, he was arrested at a park in Vienna, Virginia, after making a drop of classified documents in a plastic garbage bag for the Russians. Nearby, FBI agents discovered a bag with $50,000 in cash, intended as Hanssen’s payment. When he was arrested, Hanssen reportedly exclaimed, “What took you so long?”

In order to avoid the death penalty, Hanssen struck a deal with the government and agreed to cooperate. In July 2001, he pleaded guilty to 15 counts of espionage. The following May, he was sentenced to 15 consecutive life sentences behind bars with no possibility of parole. He spent the remainder of his life at the federal supermax prison near Florence, Colorado, along with such notorious fellow inmates include “Unabomber” Ted Kaczynski, Oklahoma City Bombing co-conspirator Terry Nichols and Ramzi Yousef, who carried out the 1993 World Trade Center bombing. Hanssen was found dead in his cell on June 5, 2023.

Hanssen wasn’t the first FBI agent arrested for spying for the Russians. In 1984, 17 years before Hanssen’s arrest, Richard Miller, a 20-year veteran who was stationed at the FBI’s foreign counterintelligence unit in Los Angeles at the time of his arrest, was arrested for selling classified documents to Russian agents, one of whom he was having an affair with. In 1986, Miller was convicted and sentenced to life in prison. His conviction later was overturned but at a second trial in 1990 he was again found guilty. Miller was released from prison in 1994.

The second FBI agent caught spying for Moscow was Earl Pitts, who volunteered to become a mole for the KGB in 1987. He handed over classified information to the Russians until 1992, by which point they’d paid him more than $220,000. In 1996, Pitts was caught in an FBI sting operation. He pleaded guilty to espionage and in 1997 was given 27 years in prison.

Robert Hanssen shares the title of one of America’s most notorious moles with Aldrich Ames. A CIA operative who spent more than 30 years with the agency and specialized in Soviet and Russian intelligence services, Ames was arrested for spying for Moscow in February 1994, almost seven years to the day before Hanssen was caught.

The son of a CIA officer, Ames began working for the CIA in the early 1960s and started selling classified information to the Soviets in 1985. Like Hanssen, he volunteered his services rather than being recruited. Apparently motivated by greed, Ames raked in some $2.5 million in illicit payments from the KGB and other Russian spy groups over the years. CIA agents grew suspicious of Ames when they noticed he was living seemingly above his means.

The FBI got involved in the case and started investigating Ames in May 1993. Following his arrest and guilty plea in 1994, he was given a life sentence with no parole. His wife was convicted of conspiring to commit espionage and received five years behind bars. As a result of Ames’ espionage more than 100 American intelligence operations were compromised and several U.S. presidents were given tainted intelligence reports.