US Ambassador in Warsaw Lee A. Feinstein said yesterday that he hopes that the publication of documents, that has long been held in possession by US authorities, proving Katyn crime was committed by the Soviet Union will encourage other governments to open their archives.

Soviet Russia only admitted to the atrocity in 1990 after blaming the Nazis for five decades.

Katyn is an important chapter in the history of Poland and World War Two, the US ambassador said, as reported by PAP Polish news agency."We hope that the decision of the United States will encourage other governments, which possess such documents, to declassify them.”

About 1,000 pages of declassified files put online by the US National Archives on Monday support the long-held suspicion that the US government covered up Soviet responsibility for the 1940 execution of Polish officers and other members of the Polish elite in the Katyn Forest, Western Russia. More than 22,000 Poles were killed then by the NKVD secret police on Joseph Stalin's orders.

The infamous massacre was kept quiet because President Franklin Delano Roosevelt did not want to anger its wartime ally Stalin, whose forces were essential to defeating Adolph Hitler.

According to the documents released Roosevelt’s administration were getting information early on from credible US sources proving who really was behind the massacre.

Two American soldiers captured by the Germans sent encrypted messages to Washington in 1943 saying they had been taken to the Katyn forest to see corpses in Polish officer uniforms and in an advanced state of decay, the declassified files showed.

The American POW also saw Polish letters, diaries, identification tags, news clippings and other objects — none dating later than the spring of 1940 — pulled from the graves.

All this suggested that that the killings must have been carried out by the Soviets, rather than the Nazis, who seized the western Russian region only in 1941.