The death squad units of the "Einsatzgruppen" were formed under the direction of Heydrich and operated by the SS before and during World War II. In September 1939, they operated in territories occupied by the German armed forces following the invasion of Poland. Men for the units were drawn from the SS, the SD, and the police. Originally part of the SiPo, in late September 1939 the operational control of the "Einsatzgruppen" was taken over by the RSHA. When the killing units were re-formed prior to the invasion of the Soviet Union in 1941, the men of the "Einsatzgruppen" were drawn from the SD, Gestapo, Kripo, Orpo, civilian (SS auxiliary) and Waffen-SS. The "Einsatzgruppen" units perpetrated atrocities in the occupied Soviet Union, including mass murder of Jews, communists, prisoners of war, and hostages, and played a key role in the Holocaust.