The Einsatzgruppen were special units of the SS during WWII that operated mainly in Eastern Europe in the early stages of Operation Barbarossa. The Einsatzgruppen (there were several sub-units, Einsatzgruppen a-d) would get in to occupied parts of the USSR before the Wermacht actually got to them,would search for Jews in villages and towns, gather them together and murder them, usually by shooting them down into mass graves. The most famous of these sites are Babi Yar (in Kiev, the Ukraine) and Ponary (outside Vilnius, Lithuania).