According to data in the possession of the Polish government-in-exile, in early 1940 the Soviet Union held as many as 15,000 Polish prisoners of war, of whom 8,300 were officers. Taken prisoner by the Red Army in the second half of September 1939, they were interned in three camps: Kozelsk, Starobelsk, and Ostaszkow. Late that year, there were reports that the three camps had been disbanded. In 1941 and 1942, the Polish government-in-exile repeatedly asked the Soviet Union for information on the prisoners' fate, but to no avail.
On April 13, 1943, the Germans announced that mass graves had been discovered in the Katyn Forest, in their area of occupation, containing the bodies of thousands of Polish officers who had been shot in the back of the head. The Germans charged the Soviet authorities with the murder and appointed a multinational medical commission to probe the matter. In May 1943, the commission reported that the graves contained the bodies of 4,143 officers, of whom 2,914 were identified by documents in their uniforms. It was the commission's opinion that the men had been shot to death in the spring of 1940. The Soviet authorities flatly rejected the accusations of the German-appointed commission, arguing that the Germans themselves had committed the deed when they had occupied the area in July 1941.
In mid-April 1943, when the Polish government-in-exile demanded that an investigation of the Katyn killings be made by the International Red Cross, the Soviet Union reacted on April 25 by severing relations with the government-in-exile. This step would have far-reaching effects on relations between the Soviet Union and Poland. In November of that year, several months after the Red Army had liberated the area, the Soviet Union appointed a commission of inquiry of its own, which blamed the Germans for the Katyn murders. A United States congressional inquiry in the early 1950s found the NKVD (the Soviet secret police) responsible, and most Western historians now believe that the massacre was committed at the behest of the Soviet authorities. On March 8, 1989, the Polish government officially accused the NKVD of perpetrating the slaughter.
In 1990, in keeping with Mikhail Gorbachev's Glasnost policy, the Soviet Union released documents indicating its responsibility for the massacre at Katyn, and uncovered further mass graves in the area.