As many as two million Jews were sent to Bełżec, Sobibór, and Treblinka to be put to death in purpose-built gas chambers.[2][3] In addition, mass-killing facilities, using Zyklon B, were developed at about the same time at the Majdanek concentration camp[2] and at Auschwitz II-Birkenau, near the earlier-established Auschwitz I camp for ethnically Polish prisoners.[4]