During a visit to Minsk in August 1941, Himmler witnessed an "Einsatzgruppen" mass execution first-hand and concluded that shooting Jews was too stressful for his men. By November he made arrangements for any SS men suffering ill health from having participated in executions to be provided with rest and mental health care. He also decided a transition should be made to gassing the victims, especially the women and children, and ordered the recruitment of expendable native auxiliaries who could assist with the murders. Gas vans, which had been used previously to kill mental patients, began to see service by all four main "Einsatzgruppen" from 1942. However, the gas vans were not popular with the "Einsatzkommandos", because removing the dead bodies from the van and burying them was a horrible ordeal. Prisoners or auxiliaries were often assigned to do this task so as to spare the SS men the trauma. Some of the early mass killings at extermination camps used carbon monoxide fumes produced by diesel engines, similar to the method used in gas vans, but by as early as September 1941 experiments were begun at Auschwitz using Zyklon B, a cyanide-based pesticide gas.