WARSAW — 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.

The declassified documents also show the United States maintaining that it couldn’t 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 U.S. 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 U.S. 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.

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It was May 1943 in the Katyn forest, a part of Russia the Germans had seized from the Soviets in 1941. A group of American and British POWs were taken against their will by their German captors to witness a horrifying scene at a clearing surrounded by pine trees: mass graves tightly packed with thousands of partly mummified corpses in well-tailored Polish officers uniforms.

The Americans — Capt. Donald B. Stewart and Lt. Col. John H. Van Vliet Jr. — hated the Nazis and didn’t want to believe the Germans. They had seen German cruelty up close, and the Soviets, after all, were their ally. The Germans were hoping to use the POWs for propaganda, and to drive a wedge between the Soviet Union and its Western Allies.

But returning to their POW camps, the Americans carried a conviction that they had just witnessed overwhelming proof of Soviet guilt. The corpses’ advanced state of decay told them the killings took place much earlier in the war, when the Soviets still controlled the area. They also saw Polish letters, diaries, identification tags, news clippings and other objects — none dated later than spring of 1940 — pulled from the graves. The evidence that did the most to convince them was the good state of the men’s boots and clothing: That told them the men had not lived long after being captured.

Stewart testified before the 1951 Congressional committee about what he saw, and Van Vliet wrote reports on Katyn in 1945 and 1950, the first of which mysteriously disappeared. But the newly declassified documents show that both sent secret encoded messages while still in captivity to army intelligence with their opinion of Soviet culpability. It’s an important revelation because it shows the Roosevelt administration was getting information early on from credible U.S. sources of Soviet guilt — yet still ignored it for the sake of the alliance with Stalin.

One shows head of Army intelligence, Gen. Clayton Bissell, confirming that some months after the 1943 visit to Katyn by the U.S. officers, a coded request by MIS-X, a unit of military intelligence, was sent to Van Vliet requesting him “to state his opinion of Katyn.” Bissell’s note said that “it is also understood Col. Van Vliet & Capt. Stewart replied.”

MIS-X was devoted to helping POWs held behind German lines escape; it also used the prisoners to gather intelligence.

A statement from Stewart dated 1950 confirms he received and sent coded messages to Washington during the war, including one on Katyn: “Content of my report was aprx (approximately): German claims regarding Katyn substantially correct in opinion of Van Vliet and myself.”

The newly uncovered documents also show Stewart was ordered in 1950 — soon before the Congressional committee began its work — never to speak about a secret message on Katyn.

Krystyna Piorkowska, author of the recently published book “English-Speaking Witnesses to Katyn: Recent Research,” discovered the documents related to the coded messages more than a week ago. She was one of several researchers who saw the material ahead of the public release.

She had already determined in her research that Van Vliet and Stewart were “code users” who had gotten messages out about other matters. But this is the first discovery of them communicating about Katyn, she said.

Another Katyn expert aware of the documents, Allen Paul, author of “Katyn: Stalin’s Massacre and the Triumph of Truth,” told the AP the find is “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 U.S. 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,” Paul 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 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 British Prime Minister 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,” O’Malley wrote, “the cumulative effect of which is to throw serious doubt on Russian disclaimers of responsibility for the massacre.”

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It wasn’t until the waning days of Soviet hegemony over Eastern Europe that reformist leader Mikhail Gorbachev publicly admitted to Soviet guilt at Katyn, a key step in Polish-Russian reconciliation.

The silence by the U.S. government has been a source of deep frustration for many Polish-Americans. One is Franciszek Herzog, 81, a Connecticut man whose father and uncle died in the massacre. After Gorbachev’s 1990 admission, he was hoping for more openness from the U.S. as well and made three attempts to obtain an apology from President George H.W. Bush.

“It will not resurrect the men,” he wrote to Bush. “But will give moral satisfaction to the widows and orphans of the victims.”

A reply he got in 1992, from the State Department, did not satisfy him. His correspondence with the government is also among the newly released documents and was obtained early by the AP from the George Bush Presidential Library.

The letter, dated Aug. 12, 1992, and signed by Thomas Gerth, then deputy director of the Office of Eastern European Affairs, shows the government stating that it lacked irrefutable evidence until Gorbachev’s admission:

“The U.S. government never accepted the Soviet Government’s claim that it was not responsible for the massacre. However, at the time of the Congressional hearings in 1951-1952, the U.S. did not possess the facts that could clearly refute the Soviets’ allegations that these crimes were committed by the Third Reich. These facts, as you know, were not revealed until 1990, when the Russians officially apologized to Poland.”

Herzog expressed frustration at that reply.

“There’s a big difference between not knowing and not wanting to know,” Herzog said. “I believe the U.S. government didn’t want to know because it was inconvenient to them.”

• Randy Herschaft reported from New York. AP reporter Monika Scislowska contributed from Warsaw.