US Air Force via Reuters

A Predator drone is shown in an undated photo from the Air Force.

Michael Reynolds / EPA

U.S. Attorney General Eric Holder addresses the National Association of Attorneys General in Washington, D.C., on Feb. 26.

The Obama administration has "no intention" of carrying out drone strikes against suspected terrorists in the United States, but could use them in response to “an extraordinary circumstance” such as the 9/11 terror attacks, according to a letter from Attorney General Eric Holder obtained by NBC News.

Sen. Rand Paul, R-Ky., who received the March 4 letter from Holder, called the attorney general’s refusal to rule out drone strikes in the U.S. “more than frightening.” 

The letter from Holder surfaced just as the Senate Intelligence Committee was voting 12-3 to approve White House counterterrorism adviser John Brennan to be CIA director. The vote came after the White House agreed to share additional classified memos on targeted drone strikes against U.S. citizens overseas.

Paul had threatened to hold up Brennan's confirmation on the floor of the Senate if the administration did not clarify whether targeted drone strikes could be used inside the U.S.

In his letter, Holder called the question of drone strikes inside the U.S. "entirely hypothetical, unlikely to occur and we hope no president will ever have to confront. … As a policy matter, moreover, we reject the use of military force where well-established law enforcement authorities in this country provide the best means for incapacitating a terrorist threat."

But Holder then appeared to leave the door open to such strikes in extreme circumstances.

Read the full letter

"It is possible, I suppose, to imagine an extraordinary circumstance in which it would be necessary and appropriate under the Constitution and applicable laws of the United States for the president to authorize the military to use lethal force within the territory of the United States. For example, the president could  conceivably have no choice but to authorize the military to use such force if necessary to protect the homeland in the circumstances of a catastrophic attack like the ones suffered on Dec. 7, 1941 and Sept. 11, 2001." 

In a statement, Paul said, “The U.S. attorney general’s refusal to rule out the possibility of drone strikes on American citizens and on American soil is more than frightening – it is an affront the Constitutional due process rights of all Americans.” 

Paul told NBC News that the response by Holder could lead to a situation where “an Arab-American in Dearborn (Mich.) is walking down the street emailing with a friend in the Mideast and all of a sudden we drop a drone” on him. He said it was “really shocking” that President Barack Obama, a former constitutional law professor, would leave the door open to such a possibility.

Paul said he will filibuster Brennan’s confirmation over the issue but acknowledged “we probably can’t stop him.” He did say, however, he intends to co-sponsor a bill with Republican Sen. Ted Cruz of Texas, to be introduced in the next few days, that would bar the president from using drone strikes in the U.S.

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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.”

But the confidential Justice Department “white paper” introduces a more expansive definition of self-defense or imminent attack than described  by Brennan or Holder in their public speeches.  It refers, for example, to what it calls a “broader concept of imminence” than actual intelligence about any ongoing plot against the U.S. homeland.    

“The condition that an operational  leader present an ‘imminent’ threat of violent attack against the United States 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,” the memo states.

Read the entire 'white paper' on drone strikes on Americans

Instead, it says,  an “informed, high-level” official of the U.S. government may determine that the targeted American  has been “recently” involved in “activities” posing a threat of a violent attack and “there is  no evidence suggesting that he has renounced or abandoned such activities.” The memo does not define “recently” or “activities.” 

As in Holder’s speech, the confidential memo lays out a three-part test that would make targeted killings of American lawful:  In addition to the suspect being an imminent threat, capture of the target must be “infeasible, and the strike must be conducted according to “law of war principles.” But the memo elaborates on some of these factors in ways that go beyond what the attorney general said publicly. For example, it states that U.S. officials may consider whether an attempted capture of a suspect  would pose an “undue risk” to U.S. personnel involved in such an operation. If so, U.S. officials could determine that the capture operation of the targeted American would not be feasible, making it lawful for the U.S. government to order a killing instead, the memo concludes.

The undated memo is entitled “Lawfulness of a Lethal Operation Directed Against a U.S. Citizen who is a Senior Operational Leader of Al Qa’ida or An Associated Force.”  It was provided to members of the Senate Intelligence and Judiciary committees in June by administration officials on the condition that it be kept confidential and  not discussed publicly.

Although not an official legal memo, the white paper was represented by administration  officials as a policy document that closely mirrors the arguments of classified memos on targeted killings by the Justice Department’s  Office of Legal Counsel, which provides authoritative legal advice to the president and all executive branch agencies. The administration has refused to turn over to Congress or release those memos publicly -- or even publicly confirm their existence. A source with access to the white paper, which is not classified, provided a copy to NBC News. 

“This is a chilling document,” said Jameel Jaffer, deputy legal director of the ACLU, which is suing to obtain administration memos about the targeted killing of Americans.  “Basically, it argues that the government has the right to carry out the extrajudicial killing of an American citizen. … It recognizes some limits on the authority it sets out, but the limits are elastic and vaguely defined, and it’s easy to see how they could be manipulated.”

In particular, Jaffer said, the memo “redefines the word imminence in a way that deprives the word of its ordinary meaning.”  

Khaled Abdullah / Reuters

Tribesmen this week examine the rubble of a building in southeastern Yemen where American teenager Abdulrahmen al-Awlaki and six suspected al-Qaida militants were killed in a U.S. drone strike on Oct. 14, 2011. Al-Awlaki, 16, was the son of Anwar al-Awlaki, who died in a similar strike two weeks earlier.

A Justice Department spokeswoman declined to comment on the white paper. The spokeswoman, Tracy Schmaler, instead pointed to public speeches by what she called a “parade” of administration officials, including Brennan, Holder, former State Department Legal Adviser Harold Koh and former Defense Department General Counsel Jeh Johnson that she said outlined the “legal framework” for such operations. 

Pressure for turning over the Justice Department memos on targeted killings of Americans appears to be building on Capitol Hill amid signs that Brennan will be grilled on the subject at his confirmation hearing before the Senate Intelligence Committee on Thursday. 

On Monday, a bipartisan group of 11 senators -- led by Democrat Ron Wyden of Oregon — wrote  a letter to President Barack Obama asking him to release all Justice Department memos on the subject. While accepting that “there will clearly be circumstances in which the president has the authority to use lethal force” against Americans who take up arms against the country,  it said, “It is vitally important ... for Congress and the American public to have a full understanding of how  the executive branch interprets the limits and boundaries of this authority.”

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The completeness of the administration’s public accounts of its legal arguments was also sharply criticized last month by U.S. Judge Colleen McMahon in response to a  lawsuit brought by the New York Times and the ACLU seeking access to the Justice Department memos on drone strikes targeting Americans under the Freedom of Information Act.  McMahon, describing herself as being caught in a “veritable Catch-22,”  said she was unable to order the release of the documents given “the thicket of laws and precedents that effectively allow the executive branch of our government to proclaim as perfectly lawful certain actions that seem on their face incompatible with our Constitution and laws while keeping the reasons for the conclusion a secret.”

In her ruling, McMahon noted that administration officials “had engaged in public discussion of the legality of targeted killing, even of citizens.” But, she wrote, they have done so “in cryptic and imprecise ways, generally without citing … any statute or court decision that justifies its conclusions.”

In one passage in Holder’s speech at Northwestern in March,  he alluded – without spelling out—that there might be circumstances where the president might order attacks against American citizens without specific knowledge of when or where an attack against the U.S. might take place.

“The Constitution does not  require the president to delay action until some theoretical end-stage of planning, when the precise time, place and manner of an attack become clear,”  he said.

But his speech did not contain the additional language in the white paper suggesting that no active intelligence about a specific attack is needed to justify a targeted strike. Similarly, Holder said in his speech that targeted killings of Americans can be justified  if “capture is not feasible.” But he did not include language in the white paper saying that an operation might not be feasible “if it could not be physically effectuated during the relevant window of opportunity or if the relevant country (where the target is located) were to decline to consent to a capture operation.” The speech also made no reference to the risk that might be posed to U.S. forces seeking to capture a target, as was  mentioned in the white paper. 

The white paper also includes a more extensive discussion of why targeted strikes against Americans does not violate constitutional protections afforded American citizens as well as   a U.S. law that criminalizes the killing of U.S. nationals overseas.

It  also discusses why such targeted killings would not be a war crime or violate a U.S. executive order banning assassinations.

 “A lawful killing in self-defense is not an assassination,” the white paper reads. “In the Department’s view, a lethal operation conducted against a U.S. citizen whose conduct poses an imminent threat of violent attack against the United States would be a legitimate act of national self-defense that would not violate the assassination ban. Similarly,  the use of lethal force, consistent with the laws of war, against an individual who is a legitimate military target would be lawful and would not violate the assassination ban.”

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Flashpoint / Evan Kohlmann

San Diego native Jehad Marwan Mustapha, at right, is believed to be the disguised English speaker who appeared in a recent video produced by the Somali Islamic terrorist group al Shabab.

The suicide bombing last weekend in Mogadishu – allegedly by a Somali American from Minnesota – has highlighted the important role played by U.S. citizens in the operations of al-Shabab, the Islamic terrorist organization battling the government in the war-torn east African nation.

FBI/AP

An FBI handout photo shows Abdisalan Hussein Ali, a Somali American who was 19 when he disappeared from Minnesota in November 2008. It is now suspected that he carried out a suicide attack against an African Union base in the Somali capital, Mogadishu, on Saturday.

If it is confirmed that Abdisalan Hussein Ali was one of two suicide bombers who attacked an African Union base, killing themselves and eight others, it will have been the third suicide bombing carried out in Somalia by Americans since 2008.

And U.S. officials tell NBC News that at least two members of the al-Shabab hierarchy are American-born, 20-something college dropouts, one of whom may be in the group’s “inner circle.”

U.S. officials and counterterrorism analysts estimate there are at least 40 Americans fighting with al-Shabab in Somalia, as well as another 200 with passports that would permit them to enter the U.S. without a visa.  

Many of the al-Shabab soldiers are Somali-Americans, many of them from the Minneapolis area, like Ali. The two leaders are not. They are Arab-Americans who traveled to Somalia in the latter part of the last decade and began rising in the ranks of the al-Qaida-linked terrorist group.

Feisal Omar / Reuters file

American born Islamic militant Omar Hammami, now known as Abu Mansoor al-Amriki, vows to avenge the death of al-Qaida leader Osama bin Laden at a news conference at a news conference farm in southern Mogadishu's Afgoye district on May 11, 2011.

One — San Diego-native Jehad Marwan Mustapha — is believed to be part of the group’s senior leadership. The other, Omar Hammami, is a unit commander.

Hammami's role has long been known. The 27-year-old from a suburb of Mobile, Ala., was profiled in the New York Times Sunday Magazine last year and has appeared in a number of al-Shabab videos, including one where he rapped an English-language recruiting pitch.

Hammami, whose father is Syrian, joined al-Shabab in late 2006 and took the name “Abu Mansoor al-Amriki,” or Abu Mansoor the American.

Photoblog: Threatening vengeance for bin Laden's death

He was interviewed in October 2007 by al-Jazeera, which identified him as a spokesman for the group, then indicted two months later by a federal grand jury on charges of material support for terrorism. He has since released four video and audio messages, most recently in April, when he mocked reports of his death and made his hip-hop recruiting pitch.

Mustapha, 29, is less well-known and has a less public role in al-Shabab, but is likely more influential in the terrorist group, according U.S. officials and Evan Kohlmann, an NBC News terrorism analyst.

“Though his name is perhaps lesser known than that of American national Omar Hammami, Jehad Mustapha is nonetheless reputed to be among the very top leaders of the foreign jihadists fighting alongside Shabab al-Mujahideen in Somalia under the banner of al-Qaida,” Kohlmann said.

One U.S. counterterrorism official, who like the others in this article spoke on condition of anonymity, referred to Mustapha as a “pretty bad dude” who has been with al-Shabab for “several years … long enough to be a significant commander … a senior player in the organization.”

Al-Shabab recently released video of a heavily masked, blue-eyed man speaking in American-accented English at a charity event in southern Somalia, and identified him as a representative of al-Qaida. Officials believe that the man could be Mustapha, who was indicted in the U.S. in August 2010 on charges of providing material support to terrorists.

In Somalia, Mustapha served under Saleh Nabhan, a senior al-Qaida and al-Shabab operative killed in a Navy SEAL operation in September 2009. 

There are conflicting reports on whether he had contact with Anwar al-Awlaki, the late New Mexico-born leading member of al-Qaida in the Arabian Peninsula, who was killed by a U.S. drone strike in Yemen in September.

Mustapha lived in San Diego through his late teens, when al-Awlaki was preaching in the city and is known to have had contact with other young jihadists, including two 9/11 hijackers, before leaving San Diego in 2002.

One U.S. official who spoke with NBC News said there is evidence to suggest contact, while declining to characterize it. A second official said he was unaware of any such connection.

Kohlmann said he also had no information indicating a connection, but added, “It’s not the most far-fetched thing I've ever heard.”

Before he was radicalized, Mustapha was working through his way through the University of California, San Diego, by manning the front desk at an auto repair shop.

By all accounts the son of working-class parents was responsible and easy-going. He was interested in business, majoring in economics.

Then, after marrying a Somali woman, he picked up and headed for her homeland. Now, say U.S. officials, when suicide bombers are dispatched to carry out their attacks, it's likely Mustapha is aware of it.