Germany had been the site of an increasing number of measures taken in the name of "racial purity" since the Nazis assumed power in 1933. Under the cover of war, the program was to be expanded to include murdering handicapped adults. Hitler would issue no law legalizing such forced "euthanasia," but was persuaded to sign this document on his personal stationery instructing his assistants Philipp Bouhler and Dr. Karl Brandt to initiate the program.
Berlin, 1 September 1939
Reichsleiter Bouhler and Dr. med. Brandt are instructed to broaden the powers of physicians designated by name, who will decide whether those who have — as far as can be humanly determined — incurable illnesses can, after the most careful evaluation, be granted a mercy death.
/signed/ Adolf Hitler
Sources:H-German Web Site