Robert Hanssen, a former FBI agent who became one of the most notorious spies in US history, died in federal prison Monday morning. He was 79.

Hanssen was found unresponsive in his cell at the United States Penitentiary Florence ADMAX in Florence, Colo. at about 6:55 a.m., according to a statement from the Federal Bureau of Prisons.

The staff tried to save his life, and requested help from emergency medical services.

But Hanssen did not survive, the statement said.

No other staff or inmates were hurt, authorities said, and there is no danger to the public.

Hanssen was serving a life sentence for spying on the United States and providing information to the Soviet Union and, later on, Russia.

During his two decades of nefarious activity, Moscow paid Hanssen for his intelligence with two Rolex watches and about $600,000 in cash and diamonds, according to Fox News.

Russia also promised that about $800,000 had been deposited in a Russian bank for Hanssen’s family.

Hanssen was apprehended in February 2001 while making a dead drop of classified information for his Russian intelligence contacts at a park near his northern Virginia home.

He pleaded guilty that July to 13 counts of espionage, one count of attempted espionage, and one of conspiracy to commit espionage.

This is a developing story.