Clayton John Lonetree was a U.S. Marine Security Guard at the American embassy in Moscow, who was recruited by the KGB after being seduced by a Soviet employee of the embassy in the early 1980s. He provided the KGB with information about embassy personnel and offices. When the KGB sought to blackmail him, he confessed to an American intelligence officer. Lonetree was convicted by a court-martial and sentenced to 15 years. He was released after serving nine.
(From: Historical Dictionary of Russian and Soviet Intelligence)