GRU
The existence of the GRU was not publicized during the Soviet era. Documents about it became available in the West in the late 1920s, and it was mentioned in the 1931 memoirs of the first OGPU defector, Georges Agabekov. Walter Krivitsky, the most senior Red Army intelligence officer ever to defect, described it in detail in his 1939 autobiography (I was Stalin's agent).