Newly declassified US memos blame the administration of President Franklin Roosevelt for covering up the massacre of Polish soldiers by Soviet leader Joseph Stalin’s Red Army forces during World War Two.

The US National Archives has released 1,000 pages relating to the Katyn Massacre – which saw Soviet secret police kill and bury 22,000 Polish soldiers in mass graves across western Russia between April and May 1940.

The archives suggest the killings were covered up to avoid antagonising Stalin, who was a US ally at the time.

The massacre was denied for decades and blamed on the Nazis until 1990 when Soviet leader Mikhail Gorbachev publicly admitted Soviet guilt.

The archives reveal a US Congressional committee, probing the massacre, blamed the Roosevelt administration, particularly pro-Soviet sympathisers, for suppressing the truth.

US officials failed to assess properly and act on “clear danger signals in Russian behaviour evident as early as 1942,” documents said.

After the Germans discovered the mass graves, they took a group of US and British prisoners of war to Katyn to see the remains.

US army documents from the archives showed two American prisoners of war had written encoded messages to intelligence officers soon after the 1943 visit, suggesting that the Soviets were responsible for the killings.

In 1945, after the end of the war, Lt Col John H. Van Vliet gave his first report to army intelligence on what he witnessed at Katyn but the report disappeared and has never been found.

After Van Vliet made a second report in 1950, the Congressional committee concluded in 1952 that there was no question that the Soviets were to blame for the massacre.

It expressed anger at the disappearance of the first Van Vliet report: “This committee believes that had the Van Vliet report been made immediately available to the Department of State and to the American public, the course of our governmental policy toward Soviet Russia might have been more realistic with more fortunate post-war results.”

The department said the documents had been declassified at the request of two US politicians, who wrote a letter to President Barack Obama in 2011 on behalf of a research body called the Katyn Council.

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