FBI agent turned notorious spy Robert Hanssen dies

One of America's most notorious spies, Robert Hanssen, was found dead on Monday at a maximum security prison in Colorado, a Bureau of Prisons spokesperson told ABC News.

Just before 7 a.m. on Monday morning, Hanssen, 79, was found dead in his cell, the official said.

The crimes Hanssen committed were some of the most dangerous and egregious in American history. Beginning in 1985, Hanssen worked with handlers from the Soviet Union, using the alias "Ramon Garcia." providing highly classified national security information in exchange for $1.4 million, according to the FBI.

Hanssen used encrypted communications, "dead drops," including one just outside of Washington, D.C., at a park in Fairfax County, Virginia, and other methods to provide information to the KGB and Soviet Union.

On more than 20 separate occasions, Hanssen left packages for Russian authorities at various drops around the Washington area, the FBI said, totaling more than 6,000 pages of classified material. Some of the material was used to hunt down confidential human sources in Russia, according to the government.

After the FBI arrested CIA analyst Aldrich Ames, who was convicted on espionage charges, the agencies realized there was a mole inside the FBI. Hanssen, who was working at the State Department when the FBI realized he was potentially a spy, was moved back to FBI headquarters and given a bogus assignment, according to details from the case.

By 2001, investigators believed Hanssen was going to make a "dead drop" of information at a park in Virginia, just outside Washington. Agents said they observed Hanssen trade a plastic bag full of classified information for $50,000 in cash.

"Robert Hanssen's death brings a somber end to one of the most infamous espionage cases in U.S. history," Javed Ali, former senior director for counterterrorism at the National Security Council, said to ABC News.

"While acting as a senior agent on the Soviet/Russian counterintelligence program at FBI headquarters, Hanssen betrayed his country for money and caused major damage to U.S. national security by revealing a significant amount of highly sensitive U.S. intelligence, the status of ongoing FBI investigations, and the identity of human sources," he said. "As reported by the media in the aftermath of his arrest, his actions led to the death of those sources in Russia or elsewhere, and most very likely made it difficult to replace their placement and access on intelligence topics for some period of time, if ever."

Hanssen pleaded guilty to 15 counts of espionage on July 6, 2001. On May 10, 2002, he was sentenced to life in prison without parole.