The memo says that because the U.S. government considered al Awlaki to be an "operational leader" of an "enemy force," it was legal for the Central Intelligence Agency to attack him with a drone even though he was a U.S. citizen.
The memo says the killing was further justified under Congressional authorization for the use of U.S. military force following the Sept. 11, 2001 hijacked-plane attacks.
The Obama administration released the memo in response to a court order following Freedom of Information lawsuits filed by the American Civil Liberties Union and The New York Times.
"High-level government officials have concluded, on the basis of al-Aulaqi's activities in Yemen, that al-Aulaqi is a leader of (Al Qaeda in the Arabian Peninsula) whose activities in Yemen pose a 'continued an imminent threat' of violence to United states persons and interests," the document said.
Awlaki was killed in what U.S. officials acknowledged at the time was a CIA drone strike in Yemen on September 30, 2011. Another American citizen, Samir Khan, was killed in the same attack, although U.S. officials have said that Khan was not intentionally targeted.
Although other Americans have appeared in Internet postings or propaganda as spokesmen or representatives for al Qaeda or its affiliates, Awlaki is the only American citizen who U.S. government officials have acknowledged was directly targeted for a U.S. drone strike.
Human rights advocates criticized the legal justification outlined in the memo as overly broad and a distortion of the law.
Pardiss Kebriaei, a lawyer with the Center for Constitutional Rights in New York, described the justification as "highly aggressive and controversial interpretations of international law."