Robert Hanssen was one of the most damaging spies in American history, but what made his case particularly disturbing was that he was an FBI counterintelligence agent whose job involved protecting American secrets from foreign intelligence services.

He joined the FBI in 1976 and began spying for the Soviet Union in November 1979, initially working with the GRU, Soviet military intelligence.

He later supplied classified information to the KGB, including the identities of American intelligence sources operating inside the Soviet Union.

At least three of those sources were executed.

The irony was that Hanssen was operating inside the very institution searching for the traitor.

After CIA officer Aldrich Ames was arrested in February 1994, investigators realised that another source was still leaking American intelligence secrets to Moscow.

The FBI and CIA established a secret joint investigation to identify the remaining mole.

Investigators initially concentrated on a CIA employee and spent nearly two years pursuing the wrong suspect.

Hanssen exploited his position and access to FBI computer systems to monitor sensitive investigations.

He continued spying intermittently until his arrest on February 18, 2001, when agents caught him making a classified-information drop in a Virginia park.

He pleaded guilty to 15 espionage charges and was sentenced to life imprisonment without parole in 2002.

He died in federal prison on June 5, 2023, aged 79.