18 U.S. Code § 1119 - Foreign murder of United States nationals(1111 refers to murder. 1112 refers to manslaughter. 1113 refers to attempted murder and manslaughter.)b) Offense.— A person who, being a national of the United States, kills or attempts to kill a national of the United States while such national is outside the United States but within the jurisdiction of another country shall be punished as provided under sections 1111, 1112, and 1113.
The text of the AUMF:
(a) IN GENERAL- That the President is authorized to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons.I haven't get read the whole memo, but so far, I don't see any consideration of a citizen's constitutional protections.
The administration's conclusion relies on the SCOTUS decision in Hamdi v. Rumsfeld which says, essentially, that the AUMF authorized the President to detain a member of the Taliban who was captured abroad in an armed conflict against the U.S. on a traditional battlefield even if he is a citizen of the United States.
The administration further concludes that since the AUMF authorizes lethal force against enemy belligerents that the AUMF authorizes the President to use lethal force against a U.S. citizen abroad who is engaged in armed conflict against the U.S.