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.

The secrecy surrounding such strikes is fast emerging as a central issue in this week’s hearing of White House counterterrorism adviser John Brennan, a key architect of the drone campaign, to be CIA director. Brennan was the first administration official to publicly acknowledge drone strikes in a speech last year, calling them “consistent with the inherent right of self-defense.” In a separate talk at the Northwestern University Law School in March, Attorney General Eric Holder specifically endorsed the constitutionality of targeted killings of Americans, saying they could be justified if government officials determine the target poses “an imminent threat of violent attack.”