Hanssen lived in a modest four-bedroom house in suburban Virginia with his wife and six children prior to his arrest.
Because of his counterintelligence role, he had access to classified information and in 1985 he started his criminal activity, sending material to Russia and the former Soviet Union.
Hanssen, who became an FBI officer on 12 January 1976, used the alias "Ramon Garcia" when corresponding with his handlers.
According to the FBI's website, he "compromised numerous human sources, counterintelligence techniques, investigations, dozens of classified US government documents, and technical operations of extraordinary importance and value".
While there was some suspicion around his unusual activities occasionally, he was not caught for years.
After the spy Aldrich Hazen Ames was arrested by the FBI in 1994, the bureau realised classified information was still being leaked, which is what instigated the investigation into Hanssen.
He had been due to retire so the FBI acted quickly in an effort to catch him "red handed".
"What we wanted to do was get enough evidence to convict him, and the ultimate aim was to catch him in the act," said Debra Evans Smith, former deputy assistant director of the Counterintelligence Division.
To lure him back to FBI headquarters for closer monitoring, he was given a fake assignment.
Hanssen began working in his new office - complete with hidden cameras and microphones - at FBI headquarters in January 2001.
A month later, investigators learned he was scheduled to make a dead drop at a park.
A dead drop is when one person leaves material for another person to later pick up at a pre-determined location, according to the Central Intelligence Agency.
