Robert Hanssen, a former FBI agent who was sentenced to life imprisonment for spying on the United States for Russia, has died after the 79-year-old was found unresponsive in his prison cell on Monday.

In 2002, Hanssen was sentenced to life imprisonment for his espionage role for Russia during and after the Cold War. The case was one of the worst espionage scandals in American history and was bad enough to earn the double agent life imprisonment without the possibility of parole.

On Monday morning, the FBI said the disgraced former agent was found dead in his cell at the United States Penitentiary Florence in Colorado also known as ADX Florence – 21 years into his life imprisonment sentence.

Several medical attempts by emergency workers to resuscitate Mr Hanssen proved unsuccessful. However, the prison authorities have yet to speak on the cause of death as at the time of filing this report.

According to the New York Times, Mr Hanssen became an agent for the FBI in 1976 and climbed up the ladder to hold several counterintelligence positions which subsequently gave him access to classified information. Then years later, he started spying for the Soviet Union.

He continued the espionage until 1980 after he was caught by his wife, Bonnie, in the basement of their house – he, however, confessed to the wife. Five years later, he again started providing information to the KGB – Russia’s own intelligence agency – before he was caught by the FBI and CIA in the year 2000.