New evidence appears to back the idea that the Roosevelt administration helped cover up Soviet guilt for the 1940 Katyn massacre of Polish soldiers.
Historians said documents, released by the US National Archives, supported the suspicion that the US did not want to anger its wartime ally, Joseph Stalin.
They showed the US was sent coded messages suggesting the Soviets, not the Nazis, carried out the massacre.
More than 22,000 Poles were killed by the Soviets on Stalin’s orders.
Soviet Russia only admitted to the atrocity in 1990 after blaming the Nazis for five decades.