Arad, Yitzhak. BELZEC, SOBIBOR, TREBLINKA - the Operation Reinhard Death Camps. Indiana University Press, 1987, pp 10-11.
A few days later an experiment with poison gas was carried out by Nebe and Dr. Widmann in Mogilev. In the local lunatic asylum, a room with twenty to thirty of the insane was closed hermetically, and two pipes were driven into the wall. A car was parked outside, and one of the metal pipes that Dr. Widmann had brought connected the exhaust of the car to the pipe in the wall. The car engine was turned on and the carbon monoxide began seeping into the room. After eight minutes, the people in the room were still alive. A second car was connected to the other pipe in the wall. The two cars were operated simultaneously, and a few minutes later all those in the room were dead.
After these experimental executions, Nebe came up with the idea of constructing a car with a hermitically sealed cabin for killing purposes. The carbon monoxide from the car's exhaust would be channeled into the sealed cabin, in which the victims stood. Nebe discussed the technical aspects of the idea with Dr. Heess and together they brought the proposal before Heydrich, who adopted it.
The Technical Department of the Reich Security Main Office, headed by SS Obersturmbannfuhrer Walter Rauff, developed a special vehicle for killing purposes. This vehicle resembled an ambulance or refrigerator truck and contained a hermetically sealed rear cabin. The victims were placed in the cabin and carbon monoxide was introduced by means of a pipe. The gassing process took between fifteen and thirty minutes. During this time the van was driven from the loading site to prepared graves.
Two types of gas vans were built: a larger one, 5.8 meters in length, and a smaller one, measuring 4.5 meters. Both were about 2.5 meters wide and 1.7 meters high. The bigger one could accommodate between 130 and 150 people, when densely packed inside, and the smaller one from 80 to 100.
The first gas vans were supplied to the Einsatzgruppen and to the Chelmno death camps in November-December 1941. The killing in Chelmno began on December 8, 1941. By the middle of 1942, about thirty gas vans had been produced by a private car manufacturer, the Gabschat Farengewerke GMBH, Will-Walter Strasse 32-38, Berlin.
Works quoted by the author:
YVA, TR-10/959, pp45-47, the trial against Dr. Widmann; Nationalsozialistiche Massentotungen, pp 81-82
Edward Serwanski, Oboz Zaglady w Chelmnie nad Narem, Poznan, 1964, p.45 Nationalsozialistische Massentotungen, p.84, gives the number of 50-60 people in the Saurer car and 25-30 people in the Diamond car.
Nationalsozialistiche Massentotungen, pp 84-86