A notorious FBI agent who spied on the United States for the Soviet Union and later Russia, Robert Hanssen, died in the Colorado maximum security prison where he was condemned to spend the remainder of his life.

Hanssen, 79, reportedly died of natural causes Monday at ADX Florence near Florence, Colorado. Federal officials said he was found unresponsive in his cell.

The infamous mole, considered one of the most prolific in U.S. intelligence history, began spying for the KGB in the 1980s while working in counterintelligence at the FBI.

Notorious spy dies

Going by the alias "Ramon Garcia," Hanssen shared a huge trove of classified information with Moscow, often conveyed in so-called "dead drops," in which agents exchange information at a pre-determined site without meeting.

Hanssen managed to evade detection for decades while living in a modest house in suburban Virginia with his wife and six kids. The unassuming spy collected over $1 million from his Russian handlers over the years.

The information he shared compromised Soviet double agents working for the U.S., some of whom were executed for treason. He also exposed a secret tunnel the FBI was building under the Russian embassy in Washington.

After the Soviet Union fell, he continued to spy for the SVR, the KGB's successor. He later said he did it for the money, although he also wrote in a letter to his handlers that he wanted to be a spy from childhood.

"I decided on this course when I was 14 years old," he wrote.

Double agent

Hanssen was finally captured in 2001 after making a "dead drop" in Virginia. Unbeknownst to Hanssen, he was being secretly monitored by the FBI at the time.

Upon his arrest he said, "What took you so long?"

He pled guilty to fifteen counts of espionage and avoided the death penalty, instead being sentenced to life in prison without parole.

"I apologize for my behavior. I am shamed by it," Hanssen said at his sentencing.

"I have opened the door for calumny against my totally innocent wife and children," he said. "I've hurt so many deeply."

His fellow inmates at the Colorado super max included notorious criminals and terrorists such as the Unabomber, El Chapo, Dhzokar Tsarnaev, and members of Al-Qaeda.