The Nazis practised their methods of genocide on people with learning disability or severe mental illness in the T4 programme.
Gas chambers disguised as showers, with nozzles that dispensed gas not water, were first used on people with learning disability.
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> 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. Experimental gassings had first been carried out at Brandenburg Prison in 1939. There, gas chambers were disguised as showers complete with fake nozzles in order to deceive victims — prototypes of the killing centers' facilities built in occupied Poland later in the war.
> Again, following procedures that would later be instituted in the extermination camps, workers removed the corpses from the chambers, extracted gold teeth, then burned large numbers of bodies together in crematoria. Urns filled with ashes were prepared in the event the family of the deceased requested the remains. Physicians using fake names prepared death certificates falsifying the cause of death, and sent letters of condolences to relatives.
> Meticulous records discovered after the war documented 70,273 deaths by gassing at the six "euthanasia" centers between January 1940 and August 1941. (This total included up to 5,000 Jews; all Jewish mental patients were killed regardless of their ability to work or the seriousness of their illness.) A detailed report also recorded the estimated savings from the killing of institutionalized patients.