The Einsatzgruppen were mobile killing units under the direction of the SS that were assigned to eliminate Jews and other perceived enemies in occupied areas, particularly in Eastern Europe. They were formed with members of the SD, the Security Police (Gestapo and KriPo), the Order Police, and Waffen-SS. They frequently received logistical support from Wehrmacht units, including the Sixth Army, and were often helped by local collaborators. In the occupied Soviet Union, they began by shooting large groups of victims. For example, in Kyiv, approximately 34,000 Jews were shot over two days in September 1941 at Babyn Yar. However, this method proved both inefficient and damaging to the mental health of those carrying it out, so SS head Heinrich Himmler ordered that a new, less personal, method be implemented. This led to the widespread use of gassing vans, in which victims were placed in the back of an airtight van which was then filled with carbon monoxide. This method, however, took longer and required removal of the bodies and cleaning of the vans. Ultimately, though the SS continued to use both of these approaches, they were superseded by transportation to industrial death camps such as Auschwitz.
Altogether, it is estimated that between one-and-a-half to two million Holocaust victims were either shot or gassed by Einsatzgruppen in the occupied territory of the Soviet Union.
Source“Einsatzgruppen: An Overview,” National Holocaust Museum