Two German officers, left, and a group of Allied officers who were prisoners of war look over a partly emptied mass grave in the Katyn Forest in May 1943.
WARSAW, Poland -- President Franklin D. Roosevelt and British Prime Minister Winston Churchill hushed up evidence that the Soviet secret police had killed thousands of Polish men in the Katyn forest in 1940 for fear of alienating World War II ally Josef Stalin, newly declassified documents show.
An estimated 22,000 Polish military officers and intellectuals were killed in the massacre at Katyn, in western Russia, many of them trucked in from prison camps, shot in the head from behind and shoved into mass graves.
The aim of the Soviets 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.
The killings continue to cast a shadow over relations between Russia and Poland, but the new documents shift the focus elsewhere: to how Washington and London put fears of upsetting the Kremlin before exposing the truth.
Instead, for years they backed the Soviet Union's version of events that Nazi Germany was behind the massacre at Katyn despite dozens of intelligence reports and witness accounts pointing to Soviet involvement.
A telegram from U.S. military intelligence dated May 28, 1943, responding to an offer of information about Katyn, put the allied position bluntly: "If you mean Katyn affair am interested only if report shows German complicity."
That telegram was among 1,000 pages of newly declassified documents and photographs that were released late Monday by the U.S. National Archives in College Park, Md.
The documents -- many of them marked secret or confidential -- included a series of exchanges between Roosevelt, Churchill and Soviet leader Stalin about reports emerging in April 1943 about the massacre.
Their concerns focused on a demand from the Polish government, in exile in London, for a Red Cross investigation into Soviet involvement in the killings, and a threat from Stalin to break off ties with the Polish government as a result.
Washington and London feared a dispute would harm the effort to defeat Nazi Germany and a letter from Roosevelt to Stalin said that Polish leader Gen. Wladyslaw Sikorski "has erred" in pressing for an investigation.
J. Scott Applewhite / AP
Col. Andrzej Kopacki, right, an assistant military attache with the Polish Embassy in Washington, attends an event on Capitol Hill to announce the release of information about the 1940 Katyn Forest massacre. At left is W.J. Milan-Kamski of Easton, Md., who is a native of Poland and World War II veteran with the Polish Army, 2nd Armored Division.
"I am inclined to think that Prime Minister Churchill will find a way of prevailing upon the Polish government in London in the future to act with more common sense," Roosevelt wrote.
Churchill made a similar point to Stalin, saying in a note he would "oppose vigorously" any Red Cross investigation.
The documents showed that London and Washington had strong evidence of Soviet involvement as early as mid-1943, soon after German forces over-ran the Katyn area and found the mass graves.
Tom Brokaw joins Morning Joe to preview a new special "Their Finest Hour – Britain in 1940-41," which looks at Britain's actions during World War II and how the country stood firm against the Nazis.
This evidence included detailed accounts from officials in the Polish exiled government and reports from U.S. diplomats stating the Polish accounts were reliable.
Divers find sunken German U-boat off Massachusetts coast
Testimony also came from an American prisoner of war, Lt. Col. John H. Van Vliet, who was taken to the massacre site by his German captors and sent coded messages back home about what he saw.
One document showed that people at the heart of the British government knew the Western allies were involved in a cover-up.
Congress honors black World War II Marines
"We have been obliged to ... restrain the Poles from putting their case clearly before the public, to discourage any attempts by the public and the press to probe the ugly story to the bottom," wrote Owen O'Malley, Britain's ambassador to the Polish government in exile, in a May 1943 letter.
"We have in fact perforce used the good name of England like the murderers used the conifers to cover up a massacre."
Poland hopes to identify remains of Auschwitz hero
Churchill passed the diplomat's candid comments on to Roosevelt in a letter, and recommended that he read them.
But in keeping with the desire at the time to keep the Katyn affair quiet, the British leader asked that Roosevelt return the document afterwards for safekeeping, saying "we are not circulating it officially in any way."
Survivors of the Blitz share their feelings and historian Juliet Gardiner describes London during the strategic, sustained bombing of Britain during World War II.
'The truth was inconvenient'
Izabella Sariusz-Skapska, president of the Katyn Families Federation, said the new documents contained new details about how much was known at the time.
"The Western allies new the exact truth about Katyn, but under war-time conditions, the truth was inconvenient."
Watch the most-viewed videos on NBCNews.com
She said she hoped the decision to declassify the U.S. documents would put pressure on the Russian government to open up its own archives about Katyn. "If there is something that we are waiting for, it is there," she said.
Members of a retirement community documented their recollections of WWII in a new book. NBC's Brian Williams reports.
White House maintained silence
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.
More Europe coverage on NBCNews.com
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.
Of all the daring escapes of World War II, the story of Gyles Mackrell and his elephants is surely one of the most unusual. Documents hidden for the best part of 70 years tell how he rescued hundreds of refugees from the Japanese invasion of Burma... with a little help from some very large friends. ITV's Sally Biddulph.
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 United States knew and when.
Complete World coverage on NBCNews.com
It was not 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."
Read more about the records relating to the Katyn massacre at the U.S. National Archives
Reuters and The Associated Press contributed to this report.
More world stories from NBC News:
Follow World News from NBCNews.com on Twitter and Facebook