From NBC:
A confidential Justice Department memo concludes that the U.S. government can order the killing of American citizens if they are believed to be “senior operational leaders” of al-Qaida or “an associated force” — even if there is no intelligence indicating they are engaged in an active plot to attack the U.S.
The 16-page memo, a copy of which was obtained by NBC News, provides new details about the legal reasoning behind one of the Obama administration’s most secretive and controversial polices: its dramatically increased use of drone strikes against al-Qaida suspects abroad, including those aimed at American citizens, such as the September 2011 strike in Yemen that killed alleged al-Qaida operatives Anwar al-Awlaki and Samir Khan. Both were U.S. citizens who had never been indicted by the U.S. government nor charged with any crimes.
While you're at it, you might want to check out this article from the Guardian, US newspapers accused of complicity as drone report reopens security debate:
On Tuesday, following Monday's disclosure by NBC of a leaked Justice Department white paper on the case for its controversial targeted killing programme, the Washington Post revealed it had previously refrained from publishing the base's location at the behest of the Obama administration over national security concerns.
The New York Times followed with its own story on the drone programme on Wednesday, and an op-ed explaining why it felt the time to publish was now.
One expert described the initial decision not to publish the base's location as "shameful and craven".