Method of mass murder used by the Nazis.
The Nazis first began using poison gas as a means for mass murder in December 1939, when an SS Sonderkommando unit used carbon monoxide to suffocate Polish mental patients. One month later, the head of the Euthanasia Program decided to use carbon monoxide to kill the handicapped, chronically ill, aged, and others who had been put in his charge. By August 1941 approximately 70 000 Germans had been murdered in five euthanasia centers, which were equipped either with stationary gas chambers or with mobile gas vans.
In the summer of 1941, the Germans commenced murdering Jews en masse in a systematic fashion. After several months, it became clear to them that the mass murder method they had previously employed, of shooting, was neither quick nor efficient enough to serve their needs. Thus, based on the experience gained in the Euthanasia Program, they began using gas chambers to annihilate European Jewry.
In December 1941, the SS inaugurated the large-scale use of gas vans at the Chelmno extermination camp. These worked by piping exhaust fumes into the enclosed vehicle through a special tube. 40 - 60 victims were jammed into the van at a time, and after several minutes, they were suffocated. However, this method was insufficient for the millions of Jews that the Nazis intended to kill. Therefore in 1942, as part of Aktion Reinhard, (the program to exterminate Jews in the Generalgouvernement) — they built three extermination camps equipped with large, stationary gas chambers. Belzec, which commenced operation in March, had three gas chambers located in a wooden barrack; Sobibor, where the killings began in May, housed its gas chambers in a brick building and Treblinka, which was established in July, had three gas chambers that could be hermetically sealed. At each of the three camps, hundreds of thousands of Jews were murdered by exhaust gas from diesel engines. During the summer and fall of 1942 the Nazis enlarged the existing gas chambers and added new ones.
When transports arrived a Sobibor, Treblinka, and Belzec, a few of the victims were chosen to join Sonderkommando units, while a few others with various skills were selected to work in repair shops which served the camp staff. The rest of the victims were sent on an assembly line, where they were stripped of their possessions and clothing and their hair was cut. They were then pushed into the gas chambers with their arms raised so the maximum number of people could be jammed in. Babies and young children were thrown in on top of the crowd. After the victims had been gassed and killed, the Sonderkommando men would remove the bodies from the chamber and bury them.
The Nazis continued to search for a more efficient method of mass murder. After some experimentation on Soviet prisoners of war, the Nazis found a commercial insecticide called Zyklon B to be an appropriate gas for their needs. Zyklon B, a form of hydrogen cyanide, was put to use in the extermination center at Auschwitz.  Over its four years of existence, more than one million people were gassed to death there. However, the Nazis were not satisfied with the rate of extermination. During the summer of 1942 plans were made to build newer, more efficient gas chambers and crematoria ovens to dispose of the corpses. The project was completed under the direction of the company JA Topf und Soehne, by the spring of 1943, allowing Auschwitz to become the Nazis' main killing center.
Some of the Nazis' other camps also contained gas chambers, but they were not used on a regular basis for mass extermination. Gas chambers functioned at Mauthausen, Neuengamme, Sachsenhausen, Stutthof, and Ravensbrueck. All of these gas chambers utilized Zyklon B to kill their victims.