The Germans became aware of the political danger associated with the mass burial of corpses in April 1943, when they discovered the graves of Polish victims of the 1940 Katyn massacre carried out by the Soviets near Smolensk. The bodies of the 10,000 Polish officers executed by the NKVD were well preserved despite their long burial. The Germans formed the Katyn Commission to prove that the Soviets were solely responsible, and used radio broadcast and newsfilm to alert the Allies to this war crime. Subsequently, the Nazi leadership, concerned about covering up their own crimes, issued the secret orders to exhume the corpses buried at death camps and burn them. The cremations began shortly after Himmler's visit to the camp in late February or early March 1943.