In early 1940 the castle was converted within just a few weeks to a Nazi euthanasia institution as part of what would later be called “Aktion T4”. The previous residents of the castle had by then been taken to other institutions in the Upper Danube Gau. They were to become the first victims of the Hartheim euthanasia centre.
The murders using carbon dioxide in the gas chamber began in May 1940. As at the other T4 murder sites, a doctor was tasked with overseeing the killing. The physician at Hartheim was Rudolf Lonauer, from Linz. Georg Renno functioned as his deputy. To ensure the smooth running and bureaucratic efficiency of the institution, a police officer was deployed as “office manager”. These managerial personnel were in charge of male and female nurses, office workers, drivers and many other staff, who were also significantly responsible for the implementation and cover-up of the killings. Between 1940 and 1944 around 30,000 people were murdered in Hartheim Castle. Some of them were people with physical and mental disabilities or psychological illnesses who had been housed in psychiatric institutions,
care facilities or homes (“Aktion T4”). Others were prisoners from the Mauthausen-Gusen, Dachau and Ravensbrück concentration camp systems who were no longer capable of work (“Aktion 14f13”, as well as “Sonderbehandlung [Special Treatment] 14f13”), along with civilian forced labourers from Eastern Europe and the Soviet Union, who were brought to Hartheim. At the turn of the year 1944/1945, demolition works were carried out in the area of the killing installations. The intention was to leave no evidence behind of how this site had been used over the previous years.