WASHINGTON - An unclassified Justice Department memo reveals that the Obama administration's justification for drone strikes that kill al-Qaida linked U.S. citizens abroad is based on a much broader definition of whether they pose an imminent threat than was previously known.

The government does not need information that a specific attack is imminent, the document says, only that the targeted suspect is involved in ongoing plotting against the United States.

"The threat posed by al-Qaida and its associated forces demands a broader concept of imminence in judging when a person continually planning terror attacks presents an imminent threat," the document says.

The undated document surfaced as Obama administration official John Brennan, who helped manage the drone program, heads to Capitol Hill on Thursday for his confirmation hearing to be CIA director. The hearing is taking place as a growing number of senators are asking to see a still-classified Justice Department legal opinion that justifies the administration's position on drones and is binding on the entire executive branch.

Controversy over U.S. policy for drone attacks mushroomed after a September 2011 drone strike in Yemen killed Anwar al-Awlaki and Samir Khan, both U.S. citizens.

In a speech last March, Attorney General Eric Holder said that in assessing when a targeted killing against a U.S. citizen is legal, the government must determine after careful review that a citizen poses an imminent threat of violent attack against the U.S.

The Justice memo says that delaying action against U.S. citizens who are linked to al-Qaida would create an unacceptably high risk because some al-Qaida leaders are continually plotting attacks on the U.S. and the U.S. may not always be aware of each specific plot as they develop.

This week, a bipartisan group of 11 senators asked President Barack Obama for all legal opinions underlying the authority to kill American citizens. The newly disclosed Justice Department document, first reported by NBC News on Monday night, represents an unclassified summary of those legal opinions and may have been prepared to deflect demands to see the actual classified legal opinions.

Sen. Dianne Feinstein, D-Calif., chairman of the Senate Intelligence Committee, said Tuesday that her panel received the unclassified document, and others, from the administration in June 2012 on a confidential basis. She said this document, coupled with other documents and closed briefings, "has allowed the Intelligence Committee to conduct appropriate and probing oversight into the use of lethal force."

Noting that she had been calling for public release of the legal basis for using lethal force, particularly against U.S. citizens, for more than a year, Feinstein said with disclosure of the white paper "the American people can review and judge the legality of these operations."

She added, however, that the committee continues to seek the actual legal opinions that provide details not contained in the white paper.

The Justice memo does require that capture of a terrorist suspect not be feasible and that any such lethal operation by the United States targeting a person comply with fundamental law-of-war principles.

"A decision maker determining whether an al-Qaida operational leader presents an imminent threat of violent attack against the United States must take into account that certain members of Al-Qaida ... are continually plotting attacks against the United States" and that "al-Qaida would engage in such attacks regularly to the extent it were able to do so," says the document.

The document also says that a decision maker must take into account that "the U.S. government may not be aware of all al-Qaida plots as they are developing and thus cannot be confident that none is about to occur; and that ... the nation may have a limited window of opportunity within which to strike in a manner that both has a high likelihood of success and reduces the probability of American casualties."

With this understanding, the document added, a high-level official could conclude, for example, that an individual poses an imminent threat of violent attack against the United States where he is an operational leader of al-Qaida or an associated force and is personally and continually involved in planning terrorist attacks against the United States.

The American Civil Liberties Union said the document is "profoundly disturbing."

"It's hard to believe that it was produced in a democracy built on a system of checks and balances," the ACLU said.

The document says that the use of lethal force would not violate the Fourth Amendment of the Constitution when a targeted person is an operational leader of an enemy force and an informed, high-level government official has determined that he poses an imminent threat of violent attack against the U.S.

The document said the courts have no role to play in the matter.

"Under the circumstances described in this paper, there exists no appropriate judicial forum to evaluate these constitutional considerations. It is well established that `matters intimately related to foreign policy, and national security are rarely proper subjects for judicial intervention,'" the document said.