In the last article I mentioned a source called "Venona Project". Here I post the origins of the Venona Project, a project that even president Franklin Delano Roosevelt never was informed about. I am afraid that is exactly the reason that it kept its secrets until the final decodifying of thousand of cables, during the eighties - hence the total contamination of FDR government with Red spies.
Here is the excerpt:
simply.the-truth: "Venona and the Cold War": the origins of the Venona Project: "The Venona Project began because Carter Clarke did not trust Joseph Stalin. Colonel Clarke was chief of the U.S Army Special Branch, part of the War Department Military Intelligence Division, and in 1943 its officer heard vague rumor of secret German_Soviet peace negotiations. With the vivid example of the August 1939 Nazi-Soviet Pact in mind, Clarke feared that a separate peace between Moscow and Berlin would allow Nazi Germany to concentrate its formidable war machine against the united States and Great Britain. Clarke thought he had a way to find out whether such negotiations were under way."