As the use of unmanned aerial drones for military purposes has intensified, the Obama administration has been working — in secret — to codify a set of rules which govern its covert campaigns in the Middle East and Asia. Now a confidential Justice Department white paper has revealed a rough legal framework for one of the drone program's most controversial aspects: the targeted killing of American citizens without due process.

According to the 16 page document obtained by NBC News, a target in these cases must be believed to be a "senior operational leader of al-Qa'ida" or an "associated force" who "poses an imminent threat" to the United States. The capture of that target must also be determined to be "infeasible," and the killing itself must be conducted in accordance with "law of war principles." But the document defines these terms vaguely and with many caveats: for example, in one section, it explains the document's broader definition of "imminence," which "does not require the United States to have clear evidence that a specific attack on U.S. persons and interests will take place in the immediate future" — in other words, says Jameel Jaffer, deputy legal director for the ACLU, it "deprives the word of its ordinary meaning."

"Targeting a member of an enemy force who poses an imminent threat to the United States is not unlawful."

It's important to note that the framework described in the document does not reflect any officially enacted policy. However, it does provide a glimpse at the Justice Department's attempts at reasoning a legal basis for the killing of American citizens as part of the Obama administration's counterterrorism campaigns in the Middle East and Asia. The issue came to a head in 2011, when two drone strikes in Yemen killed Anwar al-Awlaki, a radical Muslim cleric, and his 16 year old son, Abdulrahman — both American citizens who had never been officially charged with any crimes. John Brennan, Obama's counterterrorism czar and candidate for head of the CIA, has previously defended the killings, arguing that they are "consistent with the inherent right of self-defense."

Since then, the Obama administration has promised a clear set of guidelines for drones, and the leaked memo hints at what they might look like. However, the administration has said that even those official rules, when released, wont apply to the CIA's operations inside Pakistan's Federally Administered Tribal Areas, where a majority of the deadliest strikes have taken place.

UPDATE: During a press briefing on Tuesday, White House Press Secretary Jay Carney refused to clarify the Obama administration's justifications for drone attacks on US citizens. In response to multiple questions about the leaked DOJ memo and its expansive language, he reiterated that the administration takes the subject of targeted killings "very seriously," claiming that the drone strikes are "legal," "ethical," and "wise."