Final answer:
The first documented victims of the Nazis' use of poison gas were the disabled or mentally ill individuals killed under the T4 Euthanasia Program before the systematic extermination of the European Jews and other groups commenced.
Step-by-step explanation:
The first group of people documented to be victims of Nazi-sponsored killing through poison gas were disabled or mentally ill individuals. Even before World War II began, the Nazis had implemented a policy of 'mercy killing' in psychiatric and mental institutions where those with physical and mental disabilities were systematically murdered. With the onset of the war and after the Polish campaign, Hitler formally ordered the elimination of all 'defectives,' which led to the experimentation with mass killing techniques, including pumping carbon monoxide into sealed spaces.
The Nazis first used poison gas as a method of mass murder on these disabled or mentally ill individuals in a program that predated the establishment of the death camps. Eventually, their methodology of using gas vans that killed through carbon monoxide poisoning was expanded and refined with the use of the pesticide Zyklon B in concentration camps such as Auschwitz.