The Signals Intelligence Service (SIS) was the Army's codebreaking division headquartered at Arlington Hall. It was a part of the Signal Corps so secret that outside the office of the Chief Signal officer, it did not officially exist.
In 1943, the Army Signal Intelligence Service (later the Army Security Agency) began intercepting Soviet intelligence traffic sent mainly from New York City—assigning the code name VENONA to the project. By 1945, some 200,000 messages had been transcribed, a measure of Soviet activity. On 20 December 1946, Meredith Gardner made the first break into the VENONA code, revealing the existence of Soviet espionage at the Los Alamos National Laboratory.