The Katyn Massacre was the secret execution by the Soviets of almost 22,000 citizens of the Polish state who - after the Red Army entered Poland on 17 September 1939 - were taken prisoner or arrested. Approximately 15,000 POWs, previously held in special NKVD camps in Kozelsk, Ostashkov and Starobilsk, and 7,000 persons interned in prisons of the western district of the Ukrainian and Belarusian republic, i.e. the eastern territories of Poland included into the Soviet Union in 1939, were killed with a shot in the back of the head. The victims were mainly important citizens of the Polish state: officers of the Polish Army and the Police, officials of the state administration, and representatives of intellectual and cultural elites in Poland. They were buried anonymously in mass graves of execution: Katyn (from the Kozelsk camp), Kalinin (from the Ostashkov camp), Kharkiv (from Starobilsk camp). Those killed in Kalinin (currently Tver) were buried in Mednoye. Others, held in prisons and murdered there, were buried in previously undetermined places; two are known: in the Belarusian Republic and the Ukrainian Republic of the USSR (Kuropaty near Minsk and Bykivnia near Kiev).