The mobile killing units, or Einsatzgruppen, were made up of primarily the SS and police forces that were under the command of Heinrich Himmler. The job of these perpetrators was to eliminate political opponents of the state and those deemed racially inferior.
It is widely believed that the use of the mobile killing units was the first step of implementing the Final Solution. Following the German armed forces, these units carried out mass murder of the declared enemies of the Nazi regime. These murders were initially done through mass shootings until Himmler requested a more efficient method to be developed. As a result, mobile killing vans were invented. Victims were pushed into these sealed vans and the exhaust fumes pumped into them killed everyone inside.
By 1943, it is estimated that the mobile killing units had murdered over one million Jews in the occupied Eastern territories and tens of thousands of other enemies of the state.
For more detailed information on Mobile Killing Units:
Raul Hilberg, The Destruction of the European Jews (Yale University Press 2003; originally published 1961)