like most camps they used gas chambers to kill their victims. Birkenau had 5 gas chambers the latest could hold 2000 people. 1.3 million people were killed in Birkenau. they would tell the prisoners they war going tho take a shower and then receive warm clothes the prisoners would go in and they would seal the door. then they pumped tons of gas into the chambers the people near the vents were killed instantly the rest panicked and eventually died. the poison would condensate in there lungs and they would drown.