Where did Nazi administrators initially gain experience in mass murder?

Question

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The Nazi administrators initially gained experience in mass murder through the murder of Germans with physical and mental disabilities prior to the war. The program was called "Action T4" and took place between 1939 and 1941, resulting in the killing of tens of thousands of people in Germany. This program served as a precursor to the mass murder of Jews and other groups during the Holocaust.

Where Did Nazi Administrators Initially Gain Experience in Mass Murder