An FBI agent-turned-Russian mole who is notorious as one of the most damaging spies in US history has been found dead in prison.

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

Hanssen, 79, received more than $1.4m in cash, diamonds, and money paid into Russian accounts. Three hundred agents worked on his case.

He was sentenced in 2002 to life in prison for espionage.

A cause of death has yet to be confirmed.

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.

On 18 February 2001, Hanssen went to Foxstone Park, located in Virginia, with a plastic bag filled with classified materials.

The FBI had seen him frequent the park before and as he returned to his vehicle, he was arrested and taken into custody.

During his arrest, he asked FBI agents, "What took you so long?"

He told interrogators that the FBI security was pathetic, but he cooperated to avoid the death penalty.

Friends and neighbours said they were shocked by his arrest and described him as quiet and unassuming.

His family drove to mass every Sunday in a 10-year-old van, and was said to be a strict father, who limited television for his children.

But behind this façade lay a sexual obsession. Hanssen secretly filmed pornographic videos of his wife and showed them to a friend.

During the time of his arrest, CBS News, BBC's US partner, reported that he would frequent strip clubs where he tried to convert strippers to Catholicism.

Additionally, he would post sexually explicit stories about him and his wife online and share nude photos of her.

After growing up in Chicago, he said in a letter contained in an FBI affidavit that he was inspired by the British spy, Kim Philby.

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

He pleaded guilty to 15 counts of espionage and in May 2002 was sentenced to life without parole.

The prison, ADX Florence, is one of the most secure federal prisons in the nation, which hosts other high profile inmates like al-Qaeda terrorist Zacarias Moussaoui and Boston Marathon bomber Dzhokhar Tsarnaev.