From September to December 1939, the "Einsatzgruppen" took part in the Nazis' Action T4 program. The Nazis believed that people with physical, intellectual, and mental disabilities did not deserve to live. Action T4 was a plan to kill them all. At first, the "Einsatzgruppen" and other Nazis shot these people. However, by spring of 1940, the Nazis started using gas chambers so they could kill people with disabilities more quickly.