Einsatzgruppen (mobile killing units) were squads composed primarily of German SS and police personnel. During the invasion of the Soviet Union in June 1941, the Einsatzgruppen followed the German army as it advanced deep into Soviet territory. The Einsatzgruppen, often drawing on local civilian and police support, carried out mass-murder operations. Their victims included Jews, Roma (Gypsies), and officials of the Soviet state and the Soviet Communist party. The Einsatzgruppen also murdered thousands of residents of institutions for the mentally and physically disabled. In contrast to the methods later instituted of deporting Jews from their own towns and cities or from ghetto settings to killing centers, Einsatzgruppen came directly to the home communities of Jews and massacred them. The German army provided logistical support to the Einsatzgruppen, including supplies, transportation, housing, and occasionally manpower in the form of units to guard and transport prisoners. At first the Einsatzgruppen shot primarily Jewish men. By late summer 1941, however, wherever the Einsatzgruppen went, they shot Jewish men, women, and children without regard for age or sex, and buried them in mass graves. Often with the help of local informants and interpreters, Jews in a given locality were identified and taken to collection points. Thereafter they were marched or transported by truck to the execution site, where trenches had been prepared. In some cases the captive victims had to dig their own graves before being shot.
Shooting was the most common form of killing used by the Einsatzgruppen. Yet in the late summer of 1941, Heinrich Himmler, noting the psychological burden that mass shootings produced on his men, requested that a more convenient mode of killing be developed. The result was the gas van, a mobile gas chamber surmounted on the chassis of a cargo truck which employed carbon monoxide from the truck's exhaust to kill its victims. Gas vans made their first appearance on the eastern front in late fall 1941, and were eventually used, along with shooting, to murder Jews and other victims in most areas where the Einsatzgruppen operated. By the spring of 1943, the Einsatzgruppen and Order Police battalions had killed over a million Soviet Jews and tens of thousands of Soviet political commissars, partisans, Roma, and institutionalized disabled persons. The mobile killing methods, particularly shooting, proved to be inefficient and psychologically burdensome to the killers. Even as Einsatzgruppen units carried out their operations, the German authorities planned and began construction of special stationary gassing facilities at centralized killing centers in order to murder vast numbers of Jews.