Einsatzgruppen
Himmler decided that the answer to this problem was to kill Jews (and others) with poison gas.p. 167  Starting in 1942, all of the main Einsatzgruppen began killing people using gas vans, which had been used to kill people with disabilities early on in Action T4.  In these vans, carbon monoxide exhaust fumes from the vans' engines were pumped into the back of the truck, killing the victims inside.  However, by September 1941, Nazis at Auschwitz concentration camp had begun experimenting with Zyklon B, a cyanide-like pesticide.pp. 280–281