From the Intercept:
During the 1980s, in the final decade of the Cold War, three Americans who spied for the Soviet Union — CIA officers Edward Lee Howard and Aldrich Ames and FBI agent Robert Hanssen — are known to have helped Moscow identify, arrest, and imprison or execute most of the Soviet agents who were secretly spying for the United States. Those agent losses left the CIA virtually in the dark in Moscow for years. Ames and Hanssen were eventually caught and arrested and today remain in federal prison, while Howard defected to Moscow, where he died in 2002. Now there is growing evidence that there may have been a fourth major American spy who was never caught.