Once-top secret documents suggest US officials helped cover up a Soviet massacre of 22,000 Polish troops in 1940.
The United States National Archives released the documents on Monday.
The now declassified memos indicate American prisoners of war were taken by their German captors to Katyn three years later, where the rotting corpses of the Polish troops remained.
They sent coded messages to Washington disputing the Soviet version of events that the Nazis were behind the killings.
The material, which runs to 1,000 pages, lends weight to the theory that the US government wanted to avoid angering Joseph Stalin.
Washington was counting on its wartime ally for help in defeating Germany and Japan in World War II.
The Soviet Union only admitted guilt in 1990. The Polish troops were executed by being shot in the back of the head.
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