In 1943 the U.S. Army’s Signal Intelligence Service, the precursor to the National Security Agency made an attempt at decoding Soviet Diplomatic messages. A highly secret operation, later code named VENONA, was soon launched and but it was not until the post war 1940s that they were able to decrypt Soviet espionage messages that had been recorded but not decoded years earlier. It was suddenly revealed that Soviet Agents, amongst many other activities of interest to the United States, had been operating in Britain since the early part of the war:
20th Aug. 40
1. On 18th August I was in MORDEN in south east LONDON. The SAUSAGE-DEALERS bombed during daylight [16th] August the factories located there. The police do not permit entry into this area. Some houses near the factories have been badly smashed. From outside I could not see any particularly bad damage when I went round the other side of the factories.
2. The British FRIENDS report that the SAUSAGE-DEALERS have slightly damaged the aero-engine factory at FILTON in one of their night air raids. The majority of bombs fell in the fields near the factories.
3.The press reports that as well as ordinary bombs the SAUSAGE-DEALERS are continuing to use delayed action bombs that explode the next day.
[i] JOHN: unidentified Moscow based spymaster
[ii] SAUSAGE-DEALERS: the Germans
[iii] FRIENDS: members of Communist Party of Great Britain
[iv] DICK: unidentified Soviet Agent based in London
The VENONA operation was eventually able to identify the Soviet agents who had penetrated the MANHATTAN atom bomb program, including the Rosenbergs, but was not always able to identify known agents in sensitive positions. The operation continued until 1980 as efforts continued to identity the various individuals involved. The National Security Agency began releasing the documents to the public in 1995.