Newly released documents suggest the United States helped cover-up Soviet guilt in the massacre of Polish military officers in April 1940 at Katyn.
Historians said documents, released by the US National Archives supported the suspicion that the Roosevelt administration did not want to anger 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.
The BBC reports a review of the documents showed that American prisoners of war sent coded messages to Washington in 1943 saying they seen corpses in an advanced state of decay in the Katyn forest near Smolensk, in western Russia.
Captain Donald B Stewart and Lieutenant Colonel John Van Vliet, that the killings must have been carried out by the Soviets, rather than the Nazis, who did not occupy the area until 1941.
A statement by Captain Stewart made in 1950, confirmed he sent a coded message, the gist of which was: "German claims regarding Katyn substantially correct in opinion of Van Vliet and myself."
They were apparently persuaded by the advanced state of decay of the bodies - suggesting they must have died before August 1941, when the Germans took the area.
They also saw items found on the bodies, including letters, diaries and other items, none of which was dated later than the spring of 1940.
The state of the men's boots and clothing suggested the men had not lived long after being captured by invading Soviet forces.
A BBC reporter in Washington said it has long been believed that President Franklin Delano Roosevelt did not want to question the version of events put out by Stalin, an ally whom the Americans were counting on to defeat Germany and Japan.
The killings were carried out at Katyn and other sites by the NKVD secret police on Stalin's orders.
Members of the Polish elite, including officers, politicians and artists, were shot in the back of the head and their bodies dumped in mass graves.
