Marine Sergeant Clayton Lonetree, 25, was so highly regarded at his job as security guard at the U.S. embassy in Moscow that in November 1985 he was detached for special duty at the Reagan-Gorbachev summit in Geneva. Last week Lonetree sat in a brig at the Marine base at Quantico, Va., suspected by his superiors of helping the Soviet KGB filch classified U.S. documents from diplomatic offices in Moscow and Vienna.
Lonetree, authorities said, had an affair with a female KGB agent who was reportedly working as a translator at the embassy. The woman took him home a number of times,...