American vs. Nazi Gas Chambers Comparing Gas Chambers with Fred Leuchter

Frederick Leuchter is an consultant on the execution of criminals in the US prison system. He has constructed chambers which kill individuals convicted of capital offences, by cyanide gas.
He visits Germany and Poland, and shows that there is no evidence that the chambers said to have been built there to kill thousands of inmates at a time, in World War II, ever operated as such.