After the German invasion of the Soviet Union in 1941, Nazi SS Einsatzgruppen, or mobile killing squads, began the systematic execution of Jews in conquered territories of the U.S.S.R. These mass-killings were carried out by firing squad, but it was soon determined that this technique was inefficient.
The SS then determined that gassing, which had previously been used to kill the physically and mentally handicapped, was a more efficient means of killing large numbers of people in a short period of time. While the Operation Reinhard camps (Treblinka, Belzec, Sobibor and Chelmno) used carbon monoxide as developed in the Euthanasia program. Auschwitz and Majdanek used Zyklon B, a commercially available pesticide, after experiments on Soviet POWs and Polish prisoners had proved its efficiency.