The Obama administration's secret legal memorandum that opened the door to the killing of Anwar al-Awlaki, the U.S.-born radical Muslim cleric hiding in Yemen, found that it would be lawful only if it were not feasible to take him alive, according to people who have read the document.

The memo, written last year, came after months of extensive interagency deliberations and offers a glimpse into the legal debate that led to one of the most significant decisions made by President Obama - to move ahead with the killing of a U.S. citizen without trial.

The secret document provided the justification for acting despite an executive order banning assassinations, a federal law against murder, protections of 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 al-Awlaki's case and did not establish a broad new legal doctrine to permit the targeted killing of any Americans deemed to pose a terrorist threat.

Cleric's threat assessed

The legal analysis in essence concluded that al-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 Unites States and al Qaeda, personally posed a significant threat to Americans and Yemeni authorities were unable or unwilling to stop him.

The memorandum, which was written more than a year before al-Awlaki was killed in a drone strike last month, does not closely analyze the quality of the evidence against him.

The Obama administration has refused to acknowledge or discuss its role in the drone strike that killed al-Awlaki last month and that technically remains a covert operation. The government has also resisted growing calls that it provide a detailed public explanation of why officials deemed it lawful to kill a U.S. citizen, setting a precedent that scholars, rights activists and others say has raised concerns about the rule of law and civil liberties.

But the document that laid out the administration's justification - a roughly 50-page memorandum by the Justice Department's Office of Legal Counsel, completed around June 2010 - was described on the condition of anonymity by people who have read it.

More for you News Memo made legal case for killing

The administration did not respond to requests for comment.

The memo 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 Barron. Several news reports before June 2010 quoted anonymous counterterrorism officials as saying that al-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. Al-Awlaki was accused of helping to recruit the attacker for that operation.

Al-Awlaki, who was born in New Mexico, had also been accused of playing a role in a failed plot to bomb two cargo planes last year, part of a pattern of activities that counterterrorism officials have said showed that he had evolved from merely being a propagandist to playing an operational role in al Qaeda in the Arabian Peninsula's continuing efforts to carry out terrorist attacks.

Other assertions about al-Awlaki included that he was a leader of the group, which has become a "cobelligerent" with al Qaeda, and he was pushing it to focus on trying to attack the United States again. The lawyers were also told that capturing him alive among hostile armed allies might not be feasible if and when he were located.

Based on those premises, the Justice Department concluded that al-Awlaki was covered by the authorization to use military force against al Qaeda that Congress enacted shortly after the terrorist attacks of Sept. 11 - meaning that he was a lawful target in the armed conflict, unless some other legal prohibition trumped that authority. It then considered possible obstacles and rejected each in turn.

Killed in the strike alongside al-Awlaki was another U.S. citizen, Samir Khan, who had produced a magazine for al Qaeda in the Arabian Peninsula promoting terrorism. He was apparently not on the targeting list, making his death collateral damage. His family has issued a statement citing the Fifth Amendment and asking whether it was necessary for the government to have "assassinated two of its citizens."

"Was this style of execution the only solution?" the Khan family asked in its statement. "Why couldn't there have been a capture and trial?"

Last month, Obama's top counterterrorism adviser, John Brennan, delivered a speech in which he strongly denied the accusation that the administration had sometimes chosen to kill militants when capturing them was possible, saying the policy preference is to interrogate them for intelligence.