A former FBI agent who spied for Russia in one of the most damaging espionage betrayals in the history of the US has been found dead in jail at the age of 79.
Robert Hanssen was found dead at the ADX prison in Florence, Colorado, the Bureau announced.
Hanssen was jailed for giving Russia nuclear secrets and had served 20 years of a sentence of 15 consecutive life sentences.
A statement by the Bureau said: "Hanssen was found unresponsive at the Federal Correctional Complex (FCC) specifically, the ADX in Florence, Colorado."
Hanssen operated against the FBI, using his access to counterintelligence from 1985, sending important information to the Soviet Union, then later Russia.
The information he passed included details on his country's nuclear war preparations and a secret tunnel used for eavesdropping under the Soviet embassy in Washington DC.
It also emerged that Hanssen betrayed double agent Dmitri Polyakov after Hanssen was discovered by the FBI in 2001, who had been monitoring him for months.
They spotted Hanssen making a dead drop in a park in Virginia and his identity had been discovered after a Russian intelligence officer gave the domestic security service a file containing the traitor's prints and a recording of his voice on tape.
Polyakov, who was also betrayed by CIA mole Aldrich Ames in 1994, was later executed by Russia.
Hanssen had been in custody at ADX Florence since July 17, 2002 after he was given his life sentences in the Eastern District of Virginia.
The Bureau added: "Responding staff immediately initiated life-saving measures. Staff requested emergency medical services and life-saving efforts continued. The inmate was subsequently found dead by outside emergency medical personnel.
"The Federal Bureau of Investigation was notified. No staff or other mates were injured, and at no time was the public in danger."
Hanssen was paid well for his betrayal and earned his prison time after receiving over $1.4million paid into Russian accounts. Hanssen was also paid in cash and diamonds.
Operating under the alias 'Ramon Garcia', he "compromised numerous human sources, counterintelligence techniques, investigations, dozens of classified US government documents, and technical operations of extraordinary importance and value".
He started at the intelligence organisation in 1976 and lived a double life in Virginia with his wife and six children.
