The use of gassing as a method of mass killing originated with the so-called T4 program, the action aiming at killing people with mental and physical disabilities. When the war started, the Nazi regime saw itself in a position to realize long held policy goals with respect to people they called "non-productive eaters" because not only were these a problem in their eyes for their "hereditary material" but also because killing them in the warped logic of the Nazis would cut down on cost and that money would be used for the war effort instead.

When Hitler, Philipp Bouhler of the Reich chancellery, and Karl Brandt, Hitler's doctor, decided to bring the T4 program underway, they asked the Criminal Technical Institute of the Security Police (Kriminaltechnisches Institut der Sicherheitspolizei, KTI) what the fastest, most-effective, and time-saving method of mass killing was, they could come up with. The KTI ran several experiments, first on animals, but then in October 1939 they also experimented for the first time on mentally ill people in Posen, where about 400 people were gassed with Carbon-monoxide from gas bottles. Witnessed by August Becker, later to be the inspector for the Gas Vans in the Soviet Union, the KTI was apparently satisfied with that method, especially with how it was possible to kill a lot of people in a short amount time with a resources that was easily available because every engine could produce it. Thus they recommended this method to the planners of T4.

During 1940 the T4 program also conducted their own trials of which method of killing was the most effective. Shooting was out of the question because the idea was to keep this program as secret as possible and thus an easy and silent method had to be found. This is also why they started cremating their victims. No corpses meant no relative of one of the killed persons could insist on a pathologist examining the victims. While gas chambers and crematoria were established in six main T4 facilities -- Brandenburg, Grafeneck, Hadamar, Bernburg, Hartheim, Sonnenstein --, the worries about keeping the program secret were well founded on the side of the Nazis because when the Catholic Church started publicly preaching about the injustice of the program, the Nazi government saw itself forced to publicly stop the program. While the killing of the mentally and physically handicapped became a decentralized program with individual doctors killing their own patients instead of sending them to one of the central facilities, the gas chambers in the T4 facilities were still used for program 14f13. 14f13 was the deliberate killing of Concentration Camp inmates who had become to weak or sick to work. The name comes from the Concentration Camps' forms on causes of death. 14f13 stood for death by diseases and during some phases it can be read by historians today as proof that the people were gassed.

At the same time, the concept of killing people via gassing experienced another development. The killing of mentally and physically disabled people was not confined to Germany proper. In the territories of Poland that were annexed by Germany, a Gas Van was first used in January 1940 to kill inmates of some mental facilities near Havel. Driven by the members of the so-called Sonderkommando Lange, named after its boss Hermann Lange, this was essentially a mobile gas chamber. A grey van with a big enclosed back, it was possible to gas bottles to get Carbon-monoxide in the back of the Van. Where this van came from and who built is can not be said with certainty but it seems likely that it was something Lange himself invented in order to speed up his work of killing the handicapped people of Poland.

The decision to use gassing as the killing method in the systematic killing of Jews and Roma, the Holocaust, has several components. First, the experience of the Einsatzgruppen, who were in charge of murdering the Soviet Jews following the invasion of the USSR in June 1941. The Einsatzgruppen worked with mass shootings, first only the male Jews and from August/September 1941 on whole families and communities. When it emerged during autumn 1941 that all Jews of Europe should be killed in a systematic fashion, Himmler visited one of the mass executions. Apparently he was horrified by what he saw, especially by the impact these mass executions, sometimes taking days, had on the men of the Einsatzgruppen. He feared this would drive them into demoralization and alcoholism. So he order a method that was more humane for the executioners to be found. Once again, the leadership of the Reich Sicherheit Hauptamt turned to KTI to develop new methods of execution. After some experiments, including blowing people up with explosives, the KTI once again recommended Carbon-monoxide gassing as the "best" method to go because for the most part, it was possible to kill a lot of people relatively fast and it spared executioners having to witness the consequences of their actions for the most part.

The second factor that plays into this was that the T4 program had to be stopped around that time. So the Nazi regime suddenly found itself with about 400 people experienced with gas killings who needed a new task. From November 1941 forward a lot of the former T4 officials went on to other places with gas vans. Every Einsatzgruppe received one and there was also one in Riga, while the Sonderkommando Lange went on to construct the death camp at Chelmno/Kulmhof designed to kill the inhabitants of the Lodz Ghetto. Another gas van was also sent to Serbia, where the Wehrmacht had already killed the male Jews of the country in the course of anti-Partisan warfare and the women, children, and elderly were also slated to be murdered.

With the decision to kill all the Jews of Europe taken at some point in December 1941, the planners of the Nazi genocide found themselves with the task to kill millions of Jews in a fast, effective, and cost-effective fashion. They again decided because of their "good" experiences with the gas van on gas as the preferable method but because of the massive numbers of victims decided on stationary gas chambers. In the camps of the Aktion Reinhard, the killing of the Polish Jews from summer 1942 to spring 1943, they opted for gas chambers attached to Russian tank engines producing Carbon-monoxide. In about 9 months, they killed over 1.5 million people this way, all run by the former T4 program experts. These about 400 people managed virtually all three Reinhard Camps, Sobibor, Treblinka, and Belzec, where this took place.

Auschwitz was not a Reinhard Camp and the people in charge of Auschwitz wanted to prove that they were able to find an even better method of gassing millions of people. They came up with using Zyklon B. While more expensive than Carbon-monoxide, Zyklon B also worked faster thus increasing the capacity of the gas chambers. Furthermore because the IG Farben had its huge factory near the Auschwitz camp, ordering Zyklon B would raise no eyebrows should anyone every decided to follow that trail. With the Polish Jews all but dead, Aktion Reinhard ended and with the exception of Action Harvest Festival in the Majdanek Camp, Auschwitz became the main killing center of the Holocaust, where another million people was killed, many of them with Zyklon B.

The reason for continued cremation of the gassed victims had in part to do with secrecy as in the T4 program but also with more practical considerations, namely to prevent the spread of disease. From Treblinka, we know of the horrible story that before they had a crematorium resp. while the one they had wasn't working, the camp administration had buried the dead of Treblinka. They buried so many in fact that when the temperature started to rise in the summer, all the bile that corpses are know to produce rose above the ground and not only contaminated the camp for days but also started to flood downhill into the next village. Concerned with this, the camp administration started the bodies to be dug up again and to be burned over grids made from train tracks.

While gas chambers were in operation, gas vans also continued to be used. One case we know of is the Geheime Feldpolizei (the Gestapo of the Wehrmacht) used a gas van in 1944 in Mogilew but /u/marisacoulter is probably the person to ask about that.

So, to sum up: Gassing as a method of killing was not only used because it was cost effective but also because it was relatively cost effective while being fast and the most "humane" method when it came to the killers.

Sources:

• Mathias Beer: Die Entwicklung der Gaswagen beim Mord an den Juden. In: Vierteljahrshefte für Zeitgeschichte. Jg. 35 (1987), p. 403–417.

• Eugen Kogon, Hermann Langbein, Adalbert Rückerl (Hrsg.): Nationalsozialistische Massentötungen durch Giftgas. Frankfurt am Main 1995.

• Günter Morsch, Bertrand Perz, Astrid Ley (Hrsg.): Neue Studien zu nationalsozialistischen Massentötungen durch Giftgas: historische Bedeutung, technische Entwicklung, revisionistische Leugnung. Berlin 2011.

• Christopher R. Browning: The Path to Genocide: Essays on Launching the Final Solution.

• Raul HIlberg: The Destruction of the European Jews, 2002.

• Patrick Montague: Chelmno and the Holocaust: The History of Hitler's First Death Camp.