When questioned on the functioning of the crematoria, the judge also prevented Leuchter from testifying because "he hasn't any expertise". Leuchter also claimed that consultation relating to sodium cyanide and hydrogen cyanide with DuPont was "an on-going thing". DuPont, the largest American manufacturer of hydrogen cyanide, stated that it had "never provided any information on cyanides to persons representing themselves as Holocaust deniers, including Fred Leuchter", and had "never provided any information regarding the use of cyanide at Auschwitz, Birkenau or Majdanek."