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.
According to a review of the documents by the Associated Press, they show that American prisoners of war sent coded messages to Washington in 1943 saying they had been taken to see corpses in an advanced state of decay in the Katyn forest near Smolensk, in western Russia.
The group of American and British POWs had been taken by the Nazis against their will to witness the scene.
What they saw convinced two Americans, Capt Donald B Stewart and Lt Col 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 from one, Captain Donald B 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 seized 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.
And the good state of the men's boots and clothing suggested the men had not lived long after being captured by invading Soviet forces.
The close to 1,000 pages of new material will help determine what the US knew and when, the BBC's Kim Ghattas in Washington says.
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.
According to the report by the Associated Press, information about the massacre was suppressed at the highest levels in Washington.
Katyn expert Allen Paul told AP some of the material did not appear in the record of Congressional hearings in 1951-52 held to investigate the massacre, suggesting it had been deliberately kept hidden.
Among the new evidence is a report sent to President Roosevelt by the then British Prime Minister Winston Churchill - who did not challenge Stalin's claim either - which also pointed to Soviet guilt.
The report is written by the British ambassador to the Polish government-in-exile in London, Owen O'Malley, AP says.
"There is now available a good deal of negative evidence," Mr O'Malley wrote, "the cumulative effect of which is to throw serious doubt on Russian disclaimers of responsibility for the massacre."
The April 1940 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.