By CNN's Tim Lister

According to his Facebook profile, Abdulrahman al-Awlaki enjoyed watching The Simpsons and the BBC's Planet Earth series.

He read the Harry Potter books and listened to Snoop Dogg. He was also the 16-year-old son of the infamous al Qaeda propagandist, Anwar al-Awlaki.

And like his father he was a U.S. citizen, born in Colorado (where his father studied) on August 26, 1995.

Abdulrahman al-Awlaki left the family home in the Yemeni capital Sanaa late in September to search for his father, who was in a remote part of Yemen. By then, al-Awlaki senior had already survived at least two attempts by the U.S. to kill him. But days after Abdulrahman al-Awlaki began his quest, his father was killed in a U.S. drone strike. Two weeks later, another drone strike targeting and killing a prominent al Qaeda militant, Ibrahim al-Banna, also killed Abdulrahman al-Awlaki, a teenage cousin and several others. Abdulrahman al-Awlaki's family in Yemen confirmed that he and his cousin were killed by a a drone strike.

It's unclear why the younger al-Awlaki was with al-Banna, but the teenager's death has refocused scrutiny on the U.S. drone campaign, which has grown exponentially since being introduced in 2004 – principally in Pakistan, Afghanistan and Iraq, but more recently in Somalia, Yemen and Libya. Drones are now a controversial pillar of national security policy.

John B. Bellinger III, who was legal adviser to the U.S. State Department from 2005-09, said of the use of drones: "The U.S. spends a lot of effort attempting to minimize collateral damage and is abiding by the international law principles of distinction (ensuring a target is a lawful military target) and proportionality (the military case outweighs the risk to civilian life.)"

'Proportionality' is at the heart of the argument. Advances in drone technology have made both the surveillance and targeting of suspects more accurate. According to figures compiled by the New America Foundation on the drone campaign in Pakistan, the number of civilian deaths caused by drone attacks has dropped significantly.

But is it acceptable in law to carry out an attack against an identified terrorist suspect where others in his immediate vicinity - whose identities are unknown - are likely to be killed or injured?

Bellinger, now a partner at the law firm of Arnold & Porter in Washington, said it is acceptable, "provided that civilians are not directly targeted and that the expected damage to any civilians is proportional to the military advantage to be gained."

But who makes that judgment? And who decides which terrorist suspects are eligible for a 'kill list'?

U.S. officials have said they did not know the younger al-Awlaki was with al-Banna. But the American Civil Liberties Union has filed a Freedom of Information Act request seeking information aboutthe justification for the targeted killing of his father, and said it is deeply troubled by the fact that Abdulrahman al-Awlaki and another U.S. citizen, al Qaeda member Samir Khan, have also been killed by drone strikes in Yemen in the last month.

"The killing of three American citizens raises serious and troubling statements about whether the U.S. government was acting lawfully when it placed Anwar al-Awlaki's name on a "kill list" and when it ordered the deadly drone strikes," the ACLU asserts.

The ACLU is demanding the release of a 50-page classified memo drawn up last year by the Justice Department's Office of Legal Counsel that sought to justify the targeting of Anwar al-Awlaki.

A senior U.S. official who has seen the memo told CNN this month that it maintained Anwar al-Awlaki met the definition of a lawful target in Congress' authorization to use force against al-Qaeda enacted after 9/11, even though he was an American citizen.

He said the memo, the product of months of inter-agency discussions, argued that because he was an operational figure in al Qaeda and was planning attacks against Americans, the United States had legal justification to use force to defend itself against him. It also held that the United States had the right to take unilateral action in Yemen if Yemeni government officials were unable or unwilling to capture or kill him themselves, the official said.

A similar argument was made by President Obama's counterterrorism adviser, John Brennan, in a speech two weeks before Anwar al-Awlaki's death.

"Because we are engaged in an armed conflict with al Qaeda, the United States takes the legal position that -in accordance with international law-we have the authority to take action against al Qaeda and its associated forces without doing a separate self-defense analysis each time," he told an audience at Harvard Law School.

So important is the classified document to the way the United States carries on its campaign against alleged terrorists who hold U.S, citizenship that many politicians and legal experts have requested its publication - or at least the legal analysis that underpins it.

Jack Goldsmith, legal adviser in the Defense Department in the Bush Administration, argued on the blog Lawfare that "a thorough public explanation of the legal basis for the killing (and for targeted killings generally) would allow experts in the press, the academy, and Congress to scrutinize and criticize it."

"Such an analysis could explain, for example, whether the government believed that (Anwar) al-Awlaki possessed constitutional rights under the First, Fourth, Fifth or other amendments, and (assuming the government concluded that he possessed some such rights) why the rights were not implicated by the strike," said Goldsmith, who is now a Law professor at Harvard.

Bellinger said that U.S. citizens engaged in or suspected of terrorism are not entitled to special treatment. "If a U.S. citizen had joined the German army during World War II, the U.S. military would not have needed to seek criminal charges before killing him."

Even so, he told CNN, "the Obama administration needs to do a better job explaining who is being or has been targeted. What are the procedures being used to ensure the right people are being targeted?"

Harold Koh, current legal adviser at the U.S. State Department, set out some principles last year. "A state that is engaged in an armed conflict or in legitimate self-defense is not required to provide targets with legal process before the state may use lethal force," he told the American Society of International Law. "Our procedures and practices for identifying lawful targets are extremely robust, and advanced technologies have helped to make our targeting even more precise."

But to Bellinger, that's inadequate. "Harold Koh articulated very useful general principles about the law applicable to drone strikes, but did not provide much detail," he told CNN.

"If the U.S. wants to convince its allies and the rest of the world that it is conducting drone strikes in a lawful way, it will need to be more transparent about both programs. It does not need to disclose sensitive details, but it should provide more information about the legal rules and procedures it follows."

Brennan has argued that the U.S. justification for its expanding use of the drone program in an era of asymmetrical warfare was gaining traction. "We are finding increasing recognition in the international community that a more flexible understanding of "imminence" may be appropriate when dealing with terrorist groups, in part because threats posed by non-state actors do not present themselves in the ways that evidenced imminence in more traditional conflicts," he said recently.

Bellinger sees no evidence of that. "It's not at all clear that the international community is coming round to that. We don't see other governments acknowledging that the interpretation of 'imminence' might be more flexible in dealing with the threat of terrorism."