Under the control of Karl Brandt, "defective" children were removed from their families and taken to "hospitals" where the exterminations were carried out at the Hartheim[?] and Hadamar[?] killing centres. The program was expanded to include adults to prevent any "deficient" member of the German Master race from "breeding" so they could not pass on their inferiority.
One of the most important and well-known books about the Nazi Euthanasia action was written by Ernst Klee[?]: "Euthanasie im NS-Staat - Die Vernichtung lebensunwerten Lebens".
Klee describes the extermination "hospitals" like Grafeneck or Hartheim[?], where actually the first gas chambers were built (even before the Holocaust) and where mostly (adult) people were suffocated with Carbon monoxide. Klee describes further the killing of crippled children by doctors with lethal injections, and the starvation of patients marked for extermination.
Klee describes as well the resistance from the churches and the relatives of the victims, which led to a slow-down and greater secrecy of the operation, but did not stop it. Till the operation was conducted more covertly after August 1941, 70,000 people had already died in the gas chambers of Grafeneck, Hartheim[?], Hadamar[?], Bernburg[?], Brandenburg and Sonnenstein[?]. By that time every third inmate of a psychiatric institution in Germany had already died (leading to about 93,000 "free beds" at the end of 1941 in Nazi terminology) either by being actively killed or by starvation.
Most of the participants of the T-4 Euthanasia Program became part of the Holocaust as well, developing gas chamber "technology" and even helping to build death camps as Belzec, Treblinka or Sobibor in Operation Reinhard, as Klee notes. Aside from the well-known Auschwitz-Birkenau these were the main centers of extermination by gas for millions of people.