The idea of mass extermination with the use of stationary facilities, to which the victims were taken by train, was the result of earlier Nazi experimentation with chemically manufactured poison gas during the secretive Aktion T4 euthanasia programme against hospital patients with mental and physical disabilities. The technology was adapted, expanded, and applied in wartime to unsuspecting victims of many ethnic and national groups; the Jews were the primary target, accounting for over 90 percent of the extermination camp death toll. The genocide of the Jews of Europe was the Third Reich's "Final Solution to the Jewish question".