Aktion T4 was a postwar name coined by Allied criminal investigators for a Nazi program of mass murder of mentally and/or physically sick or severely handicapped individuals deemed incurable by authorized German physicians.
T4 is an abbreviation of Tiergartenstraße 4, a street address in the Berlin borough of Tiergarten. The building at that address was home to the Reich Chancellery department (a Nazi government) office set up in the spring of 1940, which recruited, paid and managed medical and support personnel that worked for this program. In Nazi Germany this term was never user – the program was referred to as “Gnadentod” (merciful death) or simply “Euthanasie” (euthanasia).
The first murder occurred on July 25th, 1939 three months before the program was officially established and about a month before the commencement of mass murders. Officially the program was terminated on August 24th, 1941 (when the Holocaust was already in full swing), but it continued in secret until the very last days of the Third Reich.
Hence (like in the case of the Night of the Long Knives) the legal definition of this crime was twofold: (mass) murder and conspiracy to commit murder. A substantial number of individuals who did not actually commit murders could have been charged with either felony murder (being present at the time and place of the murder) or at least accessory to murder.
Actions committed by the abovementioned perpetrators were illegal (i.e. criminal) according to German law in effect at that time because even after the German parliament passed the Reichstag Fire Decree and the Enabling Act, Adolf Hitler and his government were still bound by the Weimar Constitution still very much in effect at that time.
And the remaining articles of the Constitution (several ones were suspended by the abovementioned laws) did not allow Hitler and his cabinet to enact laws or issue orders that authorize mass murders.
After the war both Allied and West German court agreed with this legal assessment recognizing orders, decisions and actions of Adolf Hitler and the T4 personnel as capital crimes against humanity.
In December 1946 an American military tribunal began a criminal trial (commonly called the Doctors’ Trial) prosecuting 23 doctors and administrators for their roles in the T4 program. In August 1947 the American court pronounced 16 of the defendants guilty of these monstrous crimes. Seven were sentenced to death and executed on June 2nd.