An FBI agent-turned-Russian mole who was known as one of the most devastating spies in US history was discovered dead in prison.

On Monday morning, Robert Hanssen was located at a maximum-security facility in Florence, Colorado.

Hanssen, 79, received about $1.4 million in cash, jewelry, and money deposited into Russian bank accounts. Three hundred agents were assigned to his case.

In 2002, he was condemned to life in prison for espionage.

Prior to his incarceration, Hanssen resided in a modest four-bedroom house in suburban Virginia with his wife and six children.

He had access to classified information as part of his counterintelligence duty, and he began his criminal conduct in 1985, shipping material to Russia and the former Soviet Union.

Hanssen, who joined the FBI on January 12, 1976, used the identity “Ramon Garcia” for communicating with his handlers.

He “compromised numerous human sources, counterintelligence techniques, investigations, dozens of classified US government documents, and technical operations of extraordinary importance and value,” according to the FBI’s website.

While there was considerable suspicion surrounding his peculiar activities on occasion, he was not apprehended for years.

After the FBI caught spy Aldrich Hazen Ames in 1994, the bureau realized secret material was still being leaked, which prompted the inquiry into Hanssen.

He was about to retire, so the FBI proceeded quickly to catch him “red handed.”

“Our ultimate goal was to catch him in the act,” said Debra Evans Smith, former deputy assistant director of the Counterintelligence Division.

A phony assignment was assigned to him in order to entice him back to FBI headquarters for greater scrutiny.

In January 2001, Hanssen began working in his new office at FBI headquarters, complete with concealed cameras and microphones.

A month later, police discovered he was planning a dead drop in a park.

According to the Central Intelligence Agency, a dead drop occurs when one person leaves material for another person to pick up later at a predetermined place.

On February 18, 2001, Hanssen arrived at Foxstone Park in Virginia with a small bag containing classified papers.

The FBI had previously observed him visiting the park, and as he returned to his vehicle, he was arrested and taken into custody.

“What took you so long?” he asked FBI agents during his arrest.

He admitted to interrogators that the FBI’s security was appalling, but he cooperated in order to avoid the death penalty.

Friends and neighbors expressed disbelief at his detention, describing him as calm and modest.

His family travelled to mass every Sunday in a 10-year-old van, and he was believed to be a strict father who limited his children’s television viewing.

Behind this mask, though, was a sexual obsession. Hanssen covertly videotaped and showed pornographic videos of his wife to a friend.

During his arrest, CBS News, the BBC’s US partner, stated that he frequented strip clubs, where he attempted to convert strippers to Catholicism.

He would also upload naked images of his wife and post sexually graphic stories about them online.

He stated in an FBI affidavit that he was influenced by British spy Kim Philby while growing up in Chicago.

“I decided on this course when I was 14 years old,” he told his Russian handlers, according to the affidavit.

He pled guilty to 15 counts of espionage and was sentenced to life in prison without the possibility of parole in May 2002.

ADX Florence is one of the nation’s most secure federal prisons, housing high-profile convicts such as al-Qaeda terrorist Zacarias Moussaoui and Boston Marathon bomber Dzhokhar Tsarnaev.

The cause of death has yet to be determined.