Death squads: As many as 1.6 million Jews were murdered in open-air shootings by Nazis and their collaborators, before the establishment of the concentration camps. During the invasion of the Soviet Union, over 3,000 special killing units (Einsatzgruppen) followed the Wehrmacht, conducting mass murders of Poles, Communist officials, and the Jewish population that lived in Soviet territory. Poles were an early target in the AB Action, in which 30,000 Polish intellectual and political figures were rounded up, and 7,000 eventually murdered. By the summer of 1941, the Einsatzgruppen turned to targeting Jews.
Extermination Camps: These camps were dedicated entirely to mass extermination on an industrial scale, as opposed to the labor or concentration camps. Over three million Jews would die in these extermination camps. The method of killing at these camps was by poison gas, usually in "gas chambers", although many prisoners were killed in mass shootings. The majority of prisoners at these camps were not expected to live more than 24 hours beyond arrival.
In 1942, the Nazis began this most destructive phase of the Holocaust. Aktion Reinhard opened 3 extermination camps:
More than 1.7 million Jews were killed at the three Aktion Reinhard camps by October 1943. The largest death camp built was Auschwitz-Birkenau, which had both a labor camp (Auschwitz) and an extermination camp (Birkenau); the latter possessing four gas chambers and crematoria. This camp was responsible for the deaths of an estimated 1.6 million Jews 75,000 Poles and gay men, and some 19,000 Gypsies. At the peak of operations, Birkenau's gas chambers killed approximately 8,000 a day.