The US deliberately helped Russia cover up one of its most infamous Second World War atrocities to gain favour with Stalin, new documents suggest.

More than 22,000 captured Polish officers and other prisoners were systematically murdered in the Katyn forest on the western edge of Russia in 1940.

Three years later American prisoners of war sent secret coded messages to Washington with news of the massacre after seeing rows of corpses in an advanced state of decay in the forest, proof that the killers could not have been the Nazis who had only recently occupied the area.

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Their testimony 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 did not want to anger Russian leader Josef Stalin, an ally whom the Americans were counting on to defeat Germany and Japan during the war.

Documents now released lend weight to the belief that suppression within the highest levels of the US government helped cover up Soviet guilt.

The evidence is among about 1,000 pages of newly declassified documents that the United States National Archives is releasing and putting online. Historians who saw the material days before the official release described it as important and shared some highlights.

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.

The declassified documents also show the United States maintaining that it could not conclusively determine guilt until a Russian admission in 1990 - a statement that looks improbable given the huge body of evidence of Soviet guilt that had already emerged decades earlier. Historians say the new material helps to flesh out the story of what the US knew and when.

The Soviet secret police killed the 22,000 Poles with shots to the back of the head. Their aim was to eliminate a military and intellectual elite that would have put up stiff resistance to Soviet control. The men were among Poland's most accomplished - officers and reserve officers who in their civilian lives worked as doctors, lawyers, teachers, or as other professionals. Their loss has proven an enduring wound to the Polish nation.

In the early years after the war, outrage by some American officials over the concealment inspired the creation of a special US Congressional committee to investigate Katyn.

In a final report released in 1952, the committee declared there was no doubt of Soviet guilt, and called the massacre "one of the most barbarous international crimes in world history." It found that Roosevelt's administration suppressed public knowledge of the crime, but said it was out of military necessity. It also recommended the government bring charges against the Soviets at an international tribunal - something never acted upon.

Despite the committee's strong conclusions, the White House maintained its silence on Katyn for decades, showing an unwillingness to focus on an issue that would have added to political tensions with the Soviets during the Cold War.

One Katyn expert, Allen Paul, author of "Katyn: Stalin's Massacre and the Triumph of Truth," said they were "potentially explosive." He said the material does not appear in the record of the Congressional hearings in 1951-52, and appears to have also been suppressed.

He argues that the US cover-up delayed a full understanding in the United States of the true nature of Stalinism - an understanding that came only later, after the Soviets exploded an atomic bomb in 1949 and after Poland and the rest of Eastern Europe were already behind the Iron Curtain.

"The Poles had known long before the war ended what Stalin's true intentions were," he said. "The West's refusal to hear them out on the Katyn issue was a crushing blow that made their fate worse."

The historical record carries other evidence Mr Roosevelt knew in 1943 of Soviet guilt. One of the most important messages that landed on FDR's desk was an extensive and detailed report Winston Churchill sent him. Written by the British ambassador to the Polish government-in-exile in London, Owen O'Malley, it pointed to Soviet guilt at Katyn.

"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."

AP