As part of the drive by the Nazi regime to remove so-called "undesirable" elements from the German population, from September to December 1939 the Einsatzgruppen and others took part in Action T4, a program of systematic murder of persons with physical and mental disabilities and patients of psychiatric hospitals. Aktion T4 mainly took place from 1939 to 1941, but the killings continued until the end of the war. Initially the victims were shot by the Einsatzgruppen and others, but gas chambers were put into use by spring 1940.[8]