New evidence appears to lend weight to the belief that the U.S. administration helped cover up Soviet guilt for the 1940 Katyn massacre of Polish officers.
The Associated Press reported that documents made public by the U.S. National Archives on September 10 support the suspicion that the United States did not want to anger its wartime ally, Soviet leader Josef Stalin.
The documents, seen in advance by AP, reportedly show that American prisoners of war sent coded messages to Washington in 1943 to say they had seen corpses in an advanced state of decay in the Katyn Forest in western Russia.
The group of Americans had been taken by the Nazis to witness the scene.
The information shows that the deaths could not have been carried out by the Nazis, who had only recently occupied the area.
According to the AP report, the testimony about the massacre was suppressed at the highest levels in Washington.
AP quoted Katyn expert Allen Paul, who saw the documents ahead of the public release, as saying the finding was "potentially explosive."
Paul said 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.
The new evidence includes a report sent to U.S. President Franklin Roosevelt by British Prime Minister Winston Churchill, which also pointed to Soviet guilt.
AP said the report was written by Owen O'Malley, Britain's ambassador to the Polish government-in-exile in London.
"There is now available a good deal of negative evidence," O'Malley wrote, "the cumulative effect of which is to throw serious doubt on Russian disclaimers of responsibility for the massacre."
It has long been thought that Roosevelt did not want to question the version of events presented by Stalin, an ally whom the Americans were counting on to defeat Germany and Japan during World War II.
More than 20,000 members of the Polish elite, including military officers, doctors, lawyers, and teachers, were killed with shots to the back of the head and their bodies dumped in mass graves.
The April 1940 killings were carried out at Katyn and other sites by the Soviet secret police on Stalin's orders.
The Soviet Union only admitted to the atrocity in 1990 after blaming the Nazis for 50 years.
Sergei Fridinsky, Russia's chief military prosecutor, said he had not been informed about the contents of the declassified material.
Polish Foreign Minister Radoslaw Sikorski said the documents would allow future generations to better understand the traumatic experiences of Poland during World War II.