From a Washington Post obit by Ben Brasch headlined “Robert Hanssen, convicted of spying on U.S. for Moscow, dies in prison”:

Robert Hanssen, a turncoat American spy whose leaks to Moscow the FBI dubbed “possibly the worst intelligence disaster in U.S. history,” was found dead in prison Monday.

Hanssen, 79, was found unresponsive at the supermax prison in Florence, Colo., the Justice Department said.

The former FBI agent was sentenced to life in prison for espionage in 2002. His actions were so damaging that two decades later the FBI has a webpage explaining what Hanssen did and how they uncovered his ruse.