Chełmno extermination camp
During the initial phase of the Final Solution, gas vans producing poisonous exhaust fumes were developed in the occupied Soviet Union (USSR) and at the Chełmno extermination camp in occupied Poland, before being used elsewhere. The killing method was based on experience gained by the SS during the secretive "Aktion T4" programme of involuntary euthanasia. There were two types of death chambers operating during the Holocaust.