Intensification of the killing operations took place in late 1941 when the SS began construction of stationary gassing facilities to replace the use of Einsatzgruppen for mass killings.[182][183] Victims at these new extermination camps were killed with the use of carbon monoxide gas from automobile engines.[184] During Operation Reinhard, run by officers from the Totenkopfverbände, who were sworn to secrecy, three death camps were built in occupied Poland: Bełżec (operational by March 1942), Sobibór (operational by May 1942), and Treblinka (operational by July 1942),[185] with squads of Trawniki men (Eastern European collaborators) overseeing hundreds of Sonderkommando prisoners,[lower-alpha 2] who were forced to work in the gas chambers and crematoria before being murdered themselves.[186] On Himmler's orders, by early 1942 the concentration camp at Auschwitz was greatly expanded to include the addition of gas chambers, where victims were killed using the pesticide Zyklon B.[187][188]