Final answer:
The Nazis' first method used in the T4 program was transporting patients by buses and using engine exhaust gases to kill them. This process soon evolved into the use of gas chambers, with the active compound Zyklon B, at extermination camps designed for mass murder.
Step-by-step explanation:
Origins of the T4 Program's Killing Methods
The first method used by the Nazis in the T4 program to kill patients was transporting patients by buses and passing engine exhaust gases into the interior of the buses. This was part of the early phase of the action T4, which encompassed the systematic murder of individuals with disabilities and others deemed "unworthy of life" according to the Nazi ideology. Prior to the establishment of extermination camps, these gas vans were a means to efficiently murder large numbers of people.
The Nazis quickly realized that shooting victims and using gas vans placed a significant psychological burden on their personnel. Seeking more efficient and detached methods of mass murder, they began experimenting at concentration camps such as Majdanek and Auschwitz, leading to the use of the pesticide Zyklon B in gas chambers.
By early 1942, the approach to mass murder transitioned to the establishment of extermination camps specifically designed for that purpose, such as Chelmno, Belzec, Sobibor, Treblinka, Auschwitz-Birkenau, and Majdanek. These camps, distinct from concentration camps, were the sites of the overwhelming majority of killings during the Holocaust, utilizing poisoning methods including asphyxiation with poison gas and other means.