A 16-page memo obtained by NBC News Monday night, explains the apparent legal rationale for the extra-judicial killing of American citizen Anwar-al-Awlaki.

Awlaki, who was killed by an American drone strike in Yemen in September 2011, is covered, the White House memo explained, by an expansion of the previous definition of what makes an “imminent threat” to the United States.

Karen Greenberg, director of the Center on National Security at Fordham University’s School of Law, says the memo reveals that under President Barack Obama’s administration, there’s been a broad concept of what can legally be done to protect the United States in the war on terror.

“In some ways, it’s an extension of the broadening of concepts used under the Bush administration, which is to say essentially, the war on terror demands a different reading of the law, a different reading of executive power, a different reading of the enemy then we had in the past,” she said.

At the base of this controversy is were these individuals a national security threat, Greenberg says, as well as what the United States could do in its own self-defense.

“What this white paper, which is a summary of a memo, tells us is that in the name of self-defense, the United States can act in a preemptive fashion without knowing about a particular target,” she said.

If someone in the White House believes an individual has planned or is planning an attack on the United States, without specific knowledge of the plot, the president must, and can, decide on a course of action, Greenberg said.

“The problem is who do you want to give these powers to? While the Obama administration trusts itself to be judicious in its temperament and its judgment, what about others who come along?” she said.

Leaving the decision to the president is probably what should happen, Greenberg says, but eliminating a consultative process completely is troubling and disturbing to many — especially when it comes to American citizens.

“I think this is a very illuminative and comprehensive document, I think that what they have withheld is classified information, perhaps about where we were going to have drone strikes and things like that, that just wasn’t necessary for their legal justification,” she said.