Zündel and his lawyers were joined by Robert Faurisson, a French academic of literature and Holocaust denier, who came to Toronto to advise the defence, having previously testified as expert witness at the first trial. He was joined by David Irving, an English writer and Holocaust denier, who was to assist the defence and testify on Zündel's behalf. Faurisson claimed that it was technically and physically impossible for the gas chambers at Auschwitz to have functioned as extermination facilities, based on comparisons with American execution gas chambers; he therefore suggested getting an American prison warden who had participated in executions by gas to testify. Irving and Faurisson therefore invited Bill Armontrout, warden of the Missouri State Penitentiary, who agreed to testify and suggested they contact Fred A. Leuchter, a Bostonian execution equipment designer. Faurisson reported that Leuchter initially accepted the mainstream account of the Holocaust, but after two days of discussion with him, he stated that Leuchter was convinced that homicidal gassings never occurred. After having met Zündel in Toronto and agreeing to serve as an expert witness for his defence, Leuchter travelled with them to spend a week in Poland. He was accompanied by his draftsman, a cinematographer supplied by Zündel, a translator fluent in German and Polish, and his wife. Although Zündel and Faurisson did not accompany them, Leuchter said that they were with them "every step of the way" in spirit.