The program constituted the basis for the later programme of mass murder known as the Holocaust of Jews, Poles, Gypsies as well as other Slavs, and Russian POWs in camps such as Treblinka, Sobibor, Belzec, Chelmno, Auschwitz-Birkenau, and Majdanek. Gassing with Zyklon B was used mainly at Auschwitz and Majdanek with over 1 million victims, and Carbon monoxide at Treblinka, Sobibor, Belzec and Chelmno, the main extermination camps. Such mass murder followed the earlier murders through gunfire by the "Einsatzgruppen" death squads. Many of the SS staff involved in the later murders developed their lethal methods during the Action T4 programme, overseen by Conti. Victims were deceived in the same way as in Action T4, using very elaborate means to convince them that no harm was intended to them.