A new Justice Department memo on Americans and drone strikes is causing controversy.

Legal experts expressed grave reservations Tuesday about an Obama administration memo concluding that the United States can order the killing of American citizens believed to be affiliated with al-Qaida — with one saying the White House was acting as “judge, jury and executioner.”

The experts said that the memo, first obtained by NBC News, threatened constitutional rights and dangerously expanded the definition of national self-defense and of what constitutes an imminent attack.

“Anyone should be concerned when the president and his lawyers make up their own interpretation of the law or their own rules,” said Mary Ellen O’Connell, a law professor at the University of Notre Dame and an authority on international law and the use of force.

“This is a very, very dangerous thing that the president has done,” she added.

The memo, made public Monday, provides detail about the administration’s controversial expansion of drone strikes against al-Qaida suspects abroad, including those aimed at American citizens.

Among them were Anwar al-Awlaki and Samir Khan, who were killed by an American strike in September 2011 in Yemen. Both men were U.S. citizens who had not been charged with a crime.

Attorney General Eric Holder, in a talk at Northwestern University Law School in March, endorsed the constitutionality of targeted killings of Americans provided that the government determines such an individual poses “an imminent threat of violent attack.”

But the memo obtained by NBC News refers to a broader definition of imminence and specifically says the government is not required to have “clear evidence that a specific attack on U.S. persons and interests will take place in the immediate future.”

RELATED: Read the memo on drone strikes against Americans

Glenn Greenwald, a constitutional lawyer who writes about security and liberty for the British newspaper The Guardian, described the memo as “fundamentally misleading,” with a clinical tone that disguises “the radical and dangerous power it purports to authorize.”

“If you believe the president has the power to order U.S. citizens executed far from any battlefield with no charges or trial, then it’s truly hard to conceive of any asserted power you would find objectionable,” he wrote.

The attorney general told reporters Tuesday that the administration’s primary concern is to keep Americans safe, and to do it in a way consistent with American values. He said the administration was confident it was following federal and international law.

“We will have to look at this and see what it is we want to do with these memos,” he said. “But you have to understand that we are talking about things that are, that go into how we conduct our offensive operations against a clear and present danger.”

White House press secretary Jay Carney said that while the government must take the Constitution into account, U.S. citizenship does not make a leader of an enemy force immune from being targeted.

The drone strikes, and now the Justice Department memo, are expected to figure prominently Thursday when the Senate takes up the nomination of John Brennan, the White House counterterrorism adviser and architect of the drone campaign, to lead the CIA.

Sen. Ron Wyden, an Oregon Democrat, and 10 other senators wrote to President Barack Obama on Monday asking him to release all Justice Department memos on the subject.

The senators said that Congress and the public need a full understanding of how the White House views its authority so they can decide “whether the president’s power to deliberately kill American citizens is subject to appropriate limitations and safeguards.”

Jameel Jaffer, deputy legal director of the American Civil Liberties Union, described the memo as reckless. He wrote that assuming that the target of a strike is an al-Qaida leader, without court oversight, was like assuming a defendant is guilty and then asking whether a trial would be useful.

But John O. McGinnis, a professor of constitutional law at Northwestern University who worked for the White House’s Office of Legal Counsel during the Reagan and H.W. Bush administrations, said he was persuaded by the arguments in the memo, which he described as “very cautious.”

“If this is someone who has taken up affiliation with an organization attacking the United States, I don’t think it matters whether they’re a citizen — they seem to me an enemy combatant whom the president can respond to,” he said. “I think this is not a hard case.”

Sen. Dianne Feinstein of California, a Democrat and chair of the Senate Intelligence Committee, issued a statement Tuesday saying that her committee received the memo last year and wants to see other administration memos further explaining the legal framework for carrying out strikes.

At the same time, she appeared to defend the killing of al-Awlaki. She said that al-Awlaki was external operations leader of al-Qaida in the Arabian Peninsula and directed the failed attempt to blow up an airliner on Christmas Day 2009.

The memo lays out a three-part test for making targeted killings of Americans lawful. The suspect must be deemed an imminent threat, capturing the target must not be feasible, and the strike must be conducted according to “law of war principles.”

Naureen Shah, a lecturer at Columbia Law School and associate director of the Counterterrorism and Human Rights Project at the school’s Human Rights Institute, said that she was deeply troubled by the contents of the memo.

“We should be concerned when the White House is acting as judge, jury and executioner,” she said. “And there’s no one outside of the White House who has real oversight over that process. What’s put forward here is there’s no role for the courts, not even after the fact.”