A federal court released on Monday a Justice Department memorandum justifying the drone killing of Anwar al-Awlaki, but government redactions still left large swathes of the reasoning behind the operation unrevealed.

Awlaki, an American citizen who allegedly rose to become a senior member of al Qaeda's Yemen branch, was killed in a September 2011 drone strike carried out by the CIA. Journalists, human rights activists and members of Congress like Sen. Rand Paul (R-Ky.) have pushed the administration ever since to reveal its rationale for killing an American citizen without a trial.

"There is no precedent directly addressing the question in circumstances such as those present here," David Barron, a recently confirmed judge for the First Circuit Court of Appeals, writes in the July 2010 memo, acknowledging that he is in uncharted waters as he concludes that the law authorizing force against al Qaeda also justifies killing Awlaki.

The document released Monday is a 41-page memo authored by Barron, who was then acting chief of the Justice Department’s Office of Legal Counsel. It was released as part of a public records lawsuit against the government by the American Civil Liberties Union and The New York Times.

The Justice Department released a white paper last year summarizing some of the memo's key conclusions: President Barack Obama's administration believed it could kill a senior member of al Qaeda as long as he posed an "imminent" threat to the United States.

But that white paper left many of the factual questions surrounding the justification for killing Awlaki unanswered, including how the government came to conclude that he was a senior member of al Qaeda, and why he posed an imminent threat.

The memo released Monday by the Second Circuit Court of Appeals did little more to answer those questions, after the court complied with the government's request to redact any facts that would reveal intelligence or military sources and methods.

Barron's memo relies heavily on "the facts related to us" by the CIA and Department of Defense, and bases much of its legal reasoning on the veracity of statements from those government agencies.

Other, still unreleased memos may provide more information about why the Obama administration believes it had the right to kill Awlaki, and the ACLU has pledged to continue its legal fight to have more government documents released. Barron references at one point an earlier memo that explains why "we do not believe that al-Aulaqi's U.S. citizenship imposes constitutional limitations that would preclude the contemplated lethal action."

“The release of this memo represents an overdue but nonetheless crucial step towards transparency. There are few questions more important than the question of when the government has the authority to kill its own citizens,” the ACLU's deputy legal director Jameel Jaffer said in a statement.