In September 1941, SS-Obersturmbannführer Walter Rauff, head of Department II D of the Reich Security Main Office, sent for Friedrich Pradel, head of transportation service, and told him of his idea to remodel heavy trucks in such a way that they could be used to kill a greater nember of people in the Soviet territories. Rauff later declared to an official of the German embassy in Santiago, Chile.
So far as the extermination of Jews in Russia is concerned, I know that gas vans were used...I don't believe that Pradel took the initative in the development of the gas vans. He must have been under orders, either from me or my superiors. Did I think twice about employing the gas vans? I couldn't say. At the time the most important consideration for me was the psycological stress felt by the men involved in the shootings. This problem was overcome by the use of gas vans.
Sources: Kogon, Eugen, Hermann Langbein and Adalbert Ruckerl ed. Nazi Mass Murder: A Documentary History of the Use of Poison Gas. New Haven: Yale University Press, 1993, p. 53, and Electric Zen: An Einsatzgruppen Electronic Repository.