By Christian Lowe
WARSAW (Reuters) - Franklin D. Roosevelt and Winston Churchill hushed up evidence the Soviet secret police had killed thousands of Polish men in the Katyn forest in 1940 for fear of alienating World War Two 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 killings still 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 28 May, 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 on Monday by the U.S. National Archives.
The documents - many of them marked secret or confidential - included a series of exchanges between British Prime Minister Churchill, U.S. President Roosevelt, 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 row would harm the effort to defeat Nazi Germany and a letter from Roosevelt to Stalin said that Polish leader General Wladyslaw Sikorski "has erred" in pressing for an investigation.
"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.
This evidence included detailed accounts from officials in the Polish exiled government and reports from U.S. diplomats stating the Polish accounts were reliable.
Testimony also came from a U.S. 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.
"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."
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."
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."
She said she hoped the decision to de-classify 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.
(Additional reporting by Patrycja Sikora; Editing by Andrew Osborn)