The doomed were bused to killing centers in Germany and Austria--walled-in fortresses, mostly former psychiatric
hospitals, castles and a former prison--at Hartheim, Sonnenstein, Grafeneck, Bernburg, Hadamar, and Brandenburg. In the beginning, patients were killed by lethal injection. But by 1940, Hitler, on the advice of Dr. Werner Heyde, suggested that carbon monoxide gas be used as the preferred method of killing. There, gas chambers were disguised as showers complete with fake nozzles in order to deceive victims. Found on: http://fcit.usf.edu/holocaust/people/USHMMHAN.HTM bottom of page.