Radical US-Yemeni cleric Anwar al-Awlaki was targeted and killed in the drone attack in September of that year. Dual Pakistani-US citizen Samir Khan died in the same attack.
Awlaki's teenage son, Abdul Rahman, was killed in a separate US drone strike in Yemen in October 2011. None of the three US citizens were ever charged with a crime.
The New York Times and two of its journalists filed a lawsuit demanding the government make public secret papers justifying drone attacks against suspected terrorists, including American citizens.
The released July 16, 2010 legal memo from the US Department of Justice justified Awlaki's targeting on the grounds that he was an enemy leader whose capture in Yemen would have been "infeasible."
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As a leader in Al-Qaeda in the Arabian Peninsula (AQAP), his activities in Yemen posed "a continued and imminent threat" of violence to the United States, the memo said.
And, as an "active, high-level leader of an enemy force who is continually involved in planning and recruiting for terrorist attacks," Awlaki's killing would not be a war crime, it added.
His citizenship did not make him immune from being targeted abroad and the drone strike would "comply with international law," it said.
US drone strikes, a program shrouded in much secrecy, has become a key tactic in America's war against suspected terrorists in Pakistan, Somalia and Yemen.
But human rights groups say the program lacks clear legal limits.
US officials defend it as carefully regulated and one that has weakened Al-Qaeda's core leadership and reduced the threat of attack on American soil.