Zyklon B was used on 3 September 1941 in Auschwitz

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from the wiki on Zyklon B-https://en.m.wikipedia.org/wiki/Zyklon_B#:~:text=In%20early%201942%2C%20the%20Nazis,Birkenau%2C%20Majdanek%2C%20and%20elsewhere.


snip-"...In early 1942, the Nazis began using Zyklon B as the preferred killing tool in extermination camps during the Holocaust.[22] They used it to murder roughly 1.1 million people in gas chambers at Auschwitz-Birkenau, Majdanek, and elsewhere.[23][24] Zyklon B was preferred because it was assumed to be a
"humane" killing method, with the Nazis priding themselves as "civilized killers".[25] Most of the victims were Jews, and by far the majority of murders using this method took place at Auschwitz."


snip-"Rudolf Höss, commandant of Auschwitz, said that the use of Zyklon-B to murder prisoners came about on the initiative of one of his subordinates, SS-Hauptsturmführer (captain) Karl Fritzsch, who had used it to murder some Russian POWs in late August 1941 in the basement of Block 11 in the main camp. They repeated the experiment on more Russian POWs in September, with Höss watching.[34][35] Block 11 proved unsuitable, as the basement was difficult to air out afterwards and the crematorium (Crematorium I, which operated until July 1942) was some distance away."


Some engineer sat at his drawing board and carefully drew up plans to make the murder rooms look like a shower room.