In spring 1941, Reichsführer-SS Heinrich Himmler met with Reichsleiter Philipp Bouhler to discuss his desire to relieve concentration camps of sick prisoners and those no longer able to work.
Bouhler was Hitler's agent for implementation of Aktion T4, the euthanasia program for the mentally ill, disabled and inmates of hospitals and nursing homes deemed unworthy of inclusion in German Socialist society.
Himmler and Bouhler transferred technology and techniques used by Aktion T4 personnel to concentration camps and later to Einsatzgruppen and death camps, to efficiently kill unwanted prisoners and inconspicuously dispose of the bodies.
Aktion T4 was officially terminated by Hitler on August 24, 1941 but it was continued by many of the physicians who had been involved, until Socialist Germany was defeated in 1945.
-- from wiki