NSA - an earlier event

The Korean War: The SIGINT Background
ASA in the post-World War II period had broken messages used by the Soviet armed forces, police and industry, and was building a remarkably complete picture of the Soviet national security posture. It was a situation that compared favorably to the successes of World War II. Then, during 1948, in rapid succession, every one of these cipher systems went dark. Although the loss of these systems occurred over several months (and none happened at the end of a week), U.S. cryptanalysts tended to lump the disasters together under the dire designation "Black Friday."

Soviet intelligence had had an agent inside AFSA who had revealed the extent of U.S. penetration of Soviet cipher systems. This was William Weisband, who had been recruited by the KGB in 1934. During and after World War II, Weisband was involved in the U.S. COMINT efforts, working (as a native speaker of Russian) in the Russian section in ASA and, later, AFSA. Although in 1950 the FBI uncovered information alleging espionage activities by Weisband in the early 1940s, he was never charged with espionage - Weisband lost his job with AFSA and served a year in prison for contempt of a grand jury.[2]

2. Robert Louis Benson and Michael Warner, VENONA: Soviet Espionage and the American Response, 1939-1957 (NSA/CIA Publication, 1996).