gas chamber expert Fred Leuchter

Leuchter went to Auschwitz and proved that no homicidal gas chambers existed

The best known “expert analysis” of this variety is the so-called “Leuchter Report,” issued by an American manufacturer of execution equipment and proclaimed by the deniers in 1988 as a revelation on an international scale and a watershed in perceptions of Auschwitz. Leuchter states that there were far fewer compounds of hydrogen cyanide in the walls of the gas chambers at Auschwitz than in the walls of the disinfection chambers, and concludes that Zyklon B was not used to kill people, but rather, only sporadically, for disinfection.

https://auschwitz.org/en/history/holocaust-denial/leuchter-report/