The poisonous gas Zyklon B was used for the first time in the history of Auschwitz on 3 September 1941 to kill a group of 600 Soviet prisoners of war and approximately 250 sick inmates. The crime was perpetrated in the cellars of Block No. 11. As using them entailed certain “inconveniences” for the SS, especially the need to relocate the inmates living in the block for the time of the “operation”, the mortuary by the crematorium was remodelled into a gas chamber late in September. It was used to kill Soviet POWs, mostly so-called commissars, selected in the prisoner of war camps run by the Wehrmacht and brought to Auschwitz for execution.

However it is known from the testimonies of SS staff and former inmates that a number of small transports of Jews were killed in that chamber early in 1942. Where they came from has as yet not been ascertained. Probably, from the labour camps of “Organisation Schmelt” in Silesia, where they underwent selection, and the sick and exhausted were transported to Auschwitz and killed, to an extent in a form of euthanasia.

Yet the SS only used the gas chamber adjacent to Crematorium I when there was need to kill a small number of people, as its role was limited by the furnace capacity: originally burning 200 bodies a day, and later, after adding the third furnace—340 bodies a day. If far larger transports of Jews were sent for extermination, the crematorium would not be able to burn the bodies of inmates who were murdered in the camp for two or three days.

That is why, on the order of Commandant Höss, a residential house standing on the edge of woodland in Brzezinka/Birkenau, which had previously belonged to an evicted Polish family, was remodelled into a gas chamber (so-called Bunker I) in March 1942. The initial works, entailing the walling up of the windows, breaking holes in the walls for dropping Zyklon B, and installation of a powerful door had been completed by around 23 March, because on that day, a few hundred Jews were probably killed inside. It is not fully clear what procedure the SS staff applied in the extermination; all that is known is that they selected several men from among the deported; these laid the corpses removed from within the bunker in a mass grave. (It remains unknown whether the grave was ready before their arrival, or whether they had to dig it themselves.) These men were later transported to Auschwitz camp, and killed with injections of phenol into the heart.

Nor has it been possible to determine with any certainty whether the initiative of developing a murder site in Birkenau was caused more by the need to murder further Jews selected in the labour camps in Silesia, or whether it was rather connected to Höss receiving information about the intention to murder the elderly, disabled, and children from the “family transports” arriving from Slovakia in March 1942. Probably, however, Höss had been notified about the intention to send Slovak Jews to the camp, of whom only some were capable of working. Thus interpreted, the launching of the first gas chamber bunker in Birkenau as early as in March is a token that appropriate preparations had been taken earlier, and the murder of the transport of Polish Jews late in the month provided a particular trial aimed at testing its efficiency. If this was so, Höss must have been only half-satisfied with the test. For it is known that the first Sonderkommando was set up at least a month later in Birkenau. Höss must have considered that the selection of the burial work group from each transport which arrived was an impractical solution, and that appointing a group of people familiar with the nature of the work to be performed, for whom the emptying of the gas chamber would not be a psychological shock every time they performed it (as they won’t have to bury the corpses of their relatives and friends), would bring better results. It is also known that the first Sonderkommando was also employed late in April to perform certain construction works, as it received an allocation of lime and cement, most probably to install a concrete floor in the gas chamber. This would mean that the gas chamber, or rather, two chambers in Bunker I separated by a partition, were not fully operational at the time.