The mass murder of Jews was already taking place. The initial method of shooting Jews on the edges of pits was considered too time-consuming and detrimental on the mental health of the murder squads. The squads, often recruited from the local populations in conquered areas, willingly collaborated in the killings but eventually found the task gruelling. Seeking alternative methods, the Germans began experimenting with gas, using carbon monoxide in mobile units but although better this was still considered too slow and inefficient. Eventually, after experiments on Soviet prisoners of war in Auschwitz during September 1941, Zyklon B gas was discovered as a rapid and efficient means of murder.