Einsatzgruppen(Operational Squad), mobile killing squads composed of SS and police personnel which followed the German invasion of the Soviet Union in June 1941. Operating in the rear of the front lines and with the assistance of the German Army, Einsatzkommandos were deployed to organize the mass shootings of Jews, Roma (Gypsies), Communist leaders and prisoners-of-war.
The method employed was to shoot the victims in ravines, abandoned quarries or huge trenches. These murders were performed without regard for age or gender. By the spring of 1943, when the Germans began their retreat from Soviet territory, the Einsatzgruppen had murdered 1,250,000 Jews and hundreds of thousands of other Soviet nationals. The Einsatzgruppen sent back to Berlin detailed reports of their activities, which were later introduced as evidence at the Nuremberg War Crimes trials. Some of the most notorious locations of Eisatzgruppen massacres were at Babi Yar (Kiev), Ponary (Vilna), Rumbula (Riga) and Fort IX (Kovno).
Source: The Encyclopedia of the Holocaust.