Most prisoners sent to Auschwitz were selected for death. This means the SS decided they should be killed right away. Usually, the SS put all children, most women, all elderly people, people who looked sick, and people who looked like they could not work in this group. SS guards brought these people right to Auschwitz's gas chambers and killed them with poison gas. They used a form of hydrogen cyanide, a type of poison gas called a blood agent. They called this poison gas Zyklon B. This gas was a very efficient way of killing the prisoners, and could kill everyone exposed to it within 20 minutes. 