On March 4, 1948, Prüfer was interrogated by the Soviet SMERSCH Colonel-Lieutenant Dopertschuk and Colonel-Lieutenant Novikov:
»Question: A photo copy of a letter with your signature of March 2, 1943 is submitted to you, which is addressed to the SS-Bauleitung of Auschwitz and which is about ten gas testers. Explain for what purpose were these gas testers you were looking for as ordered by the SS!
This letter of March 2., 1943 has the following wording:
»We acknowledge the receipt of your telegram, as follows:
"Send immediately 10 gas testers as discussed. Submit quote later."
We herewith inform you that already two weeks ago we asked five different companies about indicating instruments for hydrocyanide-residues as per your request. Three companies have sent a negative reply and answers are still pending from the other two.
As soon as we receive word regarding this matter, we will inform you immediately so that you can contact the company that manufactures these instruments.
Heil Hitler!
i.V. Prüfer«
Answer: I looked for the gas testers, which are referred to in the photo copy of my letter of March 2, 1943, on the request of the chief of the SS-Bauleitung Bischoff , in order to furnish the gas chambers in the crematoriums with them. When Bischoff asked me with that request, he explained to me, that after the poisoning of the inmates in the gas chambers often cases happened, where even after the ventilation there were residues of hydrocyanide, which lead to the poisoning of the service personnel working in these chambers.
Therefore Bischoff asked me to find out which firms would make gas testers, which could be used to measure the concentration of hydrocyanide gasses in the gas chambers, in order to make the work of the service personnel free of danger. I could not fulfill Bischoff’s request, because I could not find a company which manufactured such gas testers.«
This is heralded by many Exterminationists as irrefutable proof for the existence of homicidal gas chambers in Auschwitz crematorium.