In 1939, with Hitler’s financial backing, a series of “euthanasia centers” began to appear.
Beginning in the fall of 1939, gassing installations were established at Bernburg, Brandenburg, Grafeneck, Hadamar, Hartheim, and Sonnenstein. Patients were selected by doctors and transferred from clinics to one of these centralized gassing installations and killed.
These “euthanasia centers” were widespread throughout Germany.
On January 9,  the first “gassing test” using carbon monoxide took place in the Brandenburg sanitarium. Between 18 and 20 people were killed, watched by psychiatrists, physicians, and nurses. In 1941, the psychiatric institution at Hadamar celebrated the cremation of its 10,000th patient where everyone—secretaries, nurses, and psychiatrists—received a bottle of beer for the occasion.
It was at these centers that the prototype for the mass killings was developed. The “patients” were gassed, and bodies cremated.
Number of victims
20 January 1940
8 February 1940
21 November 1940
6 May 1940
It is important to note that this was almost three years before the first mass killing facility, Chemlo, went into operation. When Chemlo went into operation, the killing became an industry.
No discussion of the Holocaust is complete without a discussion of Aktion T4. Many consider Aktion T4 to be the dress rehearsal for the Holocaust.
The [T4] Euthanasia Program represented in many ways a rehearsal for Nazi Germany’s subsequent genocidal policies. The Nazi leadership extended the ideological justification conceived by medical perpetrators for the destruction of the “unfit” to other categories of perceived biological enemies, most notably to Jews and Roma (Gypsies).
Planners of the “Final Solution” later borrowed the gas chamber and accompanying crematoria, specifically designed for the T4 campaign, to murder Jews in German-occupied Europe. T4 personnel who had shown themselves reliable in this first mass murder program figured prominently among the German staff stationed at the Operation Reinhard killing centers of Belzec, Sobibor, and Treblinka.
The description of the operation of Aktion T4 bears an uncomfortable resemblance to an Orwellian novel.
Beginning in October 1939, public health authorities began to encourage parents of children with disabilities to admit their young children to one of a number of specially designated pediatric clinics throughout Germany and Austria. In reality, the clinics were children’s killing wards. There, specially recruited medical staff, murdered their young charges by lethal overdoses of medication or by starvation.
There is not much more to say about this. It was methodical, cold blooded and efficient, and, under guidance of medical professions It grew into the Holocaust.