Jean-Claude Pressac (c. 1944 – 23 July 2003) was a French chemist and pharmacist by profession, who became a published authority on the Auschwitz concentration camp homicidal gas chambers deployed during the Holocaust in World War II. He is the author of the 1989 book Auschwitz: Technique and operation of the gas chambers among other publications on the subject.
Pressac was originally a Holocaust denier who, with Robert Faurisson, attempted to disprove what he considered historically inaccurate depictions of the concentration camps as extermination camps. However, upon visiting Auschwitz in 1979 and 1980, Pressac was able to view first-hand the extensive archive of original German documents thanks to the courtesy of the museum staff and administration unaware of the true purpose of his research. In Poland, Holocaust denial is punishable by law.
(Taken from Wikipedia)