Russia's Federal Archive Service, Rosarkhiv, published on 28 April on its website scanned photos of seven documents acknowledging Soviet responsibility for the massacre of 22,000 Polish officers.
One document, dated 5 March 1940, was a note from NKVD chief Lavrenty Beria. The note was signed by then-Soviet dictator Josef Stalin and three other members of the Soviet Politburo. The note ordered the execution "by firing" of Polish "nationalists and counter-revolutionaries".
For decades, Moscow blamed the Nazis for the massacre and didn't acknowledge Soviet responsibility until 1992, under the presidencies of Lech Wałęsa in Poland and Boris Yeltsin in Russia.
None of the culprits have ever been identified and investigations have been shelved. A Russian court in July 2008 refused to consider a request for a criminal investigation into the Katyń massacre.
The families of some of the victims were trying to use the Russian courts to force prosecutors to launch a new investigation into a massacre seen in Poland as a symbol of the repression the country suffered under Soviet domination.
According to the head of Rosarkhiv, Andriej Artizow, the publication of the documents has been ordered by President Dmitri Medvedev. The seven documents in question were declassified at the start of 1990s and shared with Poland, but their originals had been made available only to historians so far.
However, the Russian Communist Party still disputes the authenticity of records and maintains that the Polish officers were executed by the Nazis. Many Russians still believe that Western propaganda is placing the blame on their country for the 1939 massacre in Katyń.
The monstrosity of the Katyń massacre should be examined against the background of Stalin's purges of 1936-1938, involving large-scale physical extermination of the elite of the Communist party, the government, the army and other sectors of society.
According to the Russian Memorial Society, at least 1.7 million people were arrested and at least 724,000 were executed. Many executions were carried out in the same way as Katyń.
(EurActiv with Reuters and additional reports from the Polish press.)