WARSAW, Poland –  The American POWs sent secret coded messages to Washington with news of a Soviet atrocity: In 1943 they saw rows of corpses in an advanced state of decay in the Katyn forest, on the western edge of Russia, proof that the killers could not have been the Nazis who had only recently occupied the area.

The testimony about the infamous massacre of Polish officers might have lessened the tragic fate that befell Poland under the Soviets, some scholars believe. Instead, it mysteriously vanished into the heart of American power. The long-held suspicion is that President Franklin Delano Roosevelt didn't want to anger Josef Stalin, an ally whom the Americans were counting on to defeat Germany and Japan during World War II.

Documents released Monday and seen in advance by The Associated Press lend weight to the belief that suppression within the highest levels of the U.S. government helped cover up Soviet guilt in the killing of some 22,000 Polish officers and other prisoners in the Katyn forest and other locations in 1940.

The evidence is among about 1,000 pages of newly declassified documents that the United States National Archives is releasing Monday and putting online. Historians who saw the material days before the official release describe it as important and shared some highlights with the AP. The most dramatic revelation so far is the evidence of the secret codes sent by the two American POWs — something historians were unaware of and which adds to evidence that the Roosevelt administration knew of the Soviet atrocity relatively early on....