A massacre in Katyn forest in the western USSR. In 1943 the German army claimed to have discovered a mass grave of some 4500 Polish officers, part of a group of 15,000 Poles who had disappeared from Soviet captivity in 1940 and whose fate remained unknown. Each victim had a bullet in the base of his skull. The Soviet Union denied involvement in the massacre until April 1990, when it was confirmed that the officers had been killed in the early days of close Nazi-Soviet collaboration, by order of Stalin.