The secret document provided the justification for acting despite an executive order banning assassinations, a federal law against murder, protections in the Bill of Rights and various strictures of the international laws of war, according to people familiar with the analysis. The memo, however, was narrowly drawn to the specifics of Mr. Awlaki’s case and did not establish a broad new legal doctrine to permit the targeted killing of any Americans believed to pose a terrorist threat…

The legal analysis, in essence, concluded that Mr. Awlaki could be legally killed, if it was not feasible to capture him, because intelligence agencies said he was taking part in the war between the United States and Al Qaeda and posed a significant threat to Americans, as well as because Yemeni authorities were unable or unwilling to stop him.

The memorandum, which was written more than a year before Mr. Awlaki was killed, does not independently analyze the quality of the evidence against him…

It was principally drafted by David Barron and Martin Lederman, who were both lawyers in the Office of Legal Counsel at the time, and was signed by Mr. Barron. The office may have given oral approval for an attack on Mr. Awlaki before completing its detailed memorandum. Several news reports before June 2010 quoted anonymous counterterrorism officials as saying that Mr. Awlaki had been placed on a kill-or-capture list around the time of the attempted bombing of a Detroit-bound airliner on Dec. 25, 2009. Mr. Awlaki was accused of helping to recruit the attacker for that operation.