The 50-page memo was written last year, following months of "extensive inter-agency deliberations", by the Justice Department's Office of Legal Counsel. The Yemeni-American was killed in a drone attack in Yemen on September 30.
"The legal analysis, in essence, concluded that 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," a New York Times article said, quoting people familiar with the matter.
The secret document provided the justification for acting against the US citizen despite an executive order that bans assassinations, a federal law against murder, protections in the Bill of Rights and various provisions of the international laws of war.
The grounds that justified the killing included that Awlaki, born in New Mexico, played a role in a failed plot to bomb two cargo planes last year, gave sermons justifying violence by Muslims against the US and played an "operational role" in al-Qaeda in the Arabian Peninsula's continuing efforts to carry out terrorist attacks.