The U.S. government formally acknowledged for the first time on Wednesday that it had killed four Americans, including cleric Anwar al-Awlaki, who died in drone strikes in Yemen and Pakistan.
Attorney General Eric Holder named the dead U.S. citizens in a letter to members of Congress a day before President Barack Obama is expected to promise more transparency on national security issues in a speech on counterterrorism.
A White House official said Obama would lay out in his speech why the use of drones is "necessary, legal and just."
His speech would coincide with the signing of new presidential policy guidance setting out standards for U.S. drone strikes, the official said.
Reuters reported earlier this week that the administration had decided to give the Pentagon control of some drone operations that are now run by the CIA. Such a move would put the use of unmanned aerial vehicles under greater congressional oversight.
In his letter to members of Congress, Holder confirmed that the United States had "specifically targeted and killed" New Mexico-born Awlaki in Yemen in 2011.
He said the other three Americans who were killed, but not deliberately targeted, were Awlaki's teenage son Abdulrahman, Samir Khan, an American of Pakistani origin who died in Yemen, and Jude Kenan Mohammed from North Carolina, who was indicted on U.S. terrorism charges in 2009 and was killed in Pakistan.
U.S. officials had previously acknowledged privately that the two Awlakis and Khan had been killed in drone strikes. Mohammed's death was also in a drone strike, a source familiar with the matter said.
Holder's letter offered a detailed justification for the CIA'S killing of Awlaki, who Holder said had "repeatedly made clear his intent to attack U.S. persons and his hope that these attacks would take American lives."
The killing of Awlaki, 40, was the first, and only, publicly known case in which a U.S. citizen was deliberately targeted for killing by drone strike.
Holder offered considerable new detail on the government's rationale for adding the U.S.-born preacher's name to a kill list that previously had been exclusively reserved for non-U.S. citizens.
"Based on generations-old legal principles and Supreme Court decisions handed down during World War II, as well as during the current conflict, it is clear and logical that United States citizenship alone does not make such individuals immune from being targeted," Holder said.
Obama is under pressure from a series of scandals which critics say shows his administration is secretive and weak on protecting civil liberties.
The Internal Revenue Service official at the center of a scandal about extra tax scrutiny of conservative groups told Congress on Wednesday she had done nothing wrong but invoked her constitutional right not to answer questions.
Holder said the White House wanted counterterrorism efforts to "become more transparent to the American people and to the world."
Awlaki's killing was approved by a senior committee of Obama administration officials. The president himself did not have to approve adding Awlaki's name to the target list, officials said at the time, but he would have been notified of the senior officials' decision. If Obama had objected, the decision would have been nullified.