– One day before President Obama is due to deliver a major speech on national security, his administration on Wednesday formally acknowledged that the United States had killed four U.S. citizens in drone strikes in Yemen and Pakistan.

In a letter to congressional leaders obtained by the New York Times, Attorney General Eric Holder disclosed that the administration had deliberately killed Anwar al-Awlaki, a radical Muslim cleric who was killed in a drone strike in September 2011 in Yemen.

The U.S. responsibility for Al-Awlaki's death has been widely reported, but the administration had until now refused to confirm or deny it.

The letter also said that the United States had killed three other Americans: Samir Khan, who was killed in the same strike; Al-Awlaki's son Abdulrahman al-Awlaki, who was also killed in Yemen; and Jude Mohammed, who was killed in a strike in Pakistan.

"These individuals were not specifically targeted by the United States," Holder wrote.

While rumors of Mohammed's death had appeared in local news reports in Raleigh, N.C., where he lived, his death had not been confirmed by the U.S. government until Wednesday.

According to former acquaintances of Mohammed in North Carolina, he appears to have been killed in a November 2011 drone strike in South Waziristan, in Pakistan's tribal area. Mohammed's wife, whom he had met and married in Pakistan, subsequently called his mother in North Carolina to tell her of his death, the friends say.

Mohammad was one of eight men who remained on the loose after they were indicted by a federal grand jury in North Carolina in July 2009 on charges of plotting to attack the Marine Corps base at Quantico, Va. According to the indictment, Mohammad "departed the United States to travel to Pakistan to engage in violent jihad" in October 2008.

Holder, in a speech at Northwestern University Law School last year, laid out the administration's basic legal thinking that U.S. citizens who are deemed to be operational terrorists, who pose an "imminent threat of violent attack" and whose capture is infeasible may be targeted. That abstract legal thinking — including an elastic definition of what counts as "imminent" — was further laid out in an unclassified white paper provided to Congress last year, which was leaked earlier this year.

But Holder's letter to Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., went further in discussing the death of al-Awlaki in particular, an operation the administration had previously refused to publicly acknowledge. He said it was not Al-Awlaki's words urging violent attacks against Americans that led the United States to target him, but direct actions in planning attacks.

Holder claimed that Al-Awlaki not only "planned" the attempted bombing of a Detroit-bound airliner on Dec. 25, 2009, a claim that has been widely discussed in court documents and elsewhere, but also "played a key role" in an October 2010 plot to bomb cargo planes bound for the United States, including taking "part in the development and testing" of the bombs.

"Moreover, information that remains classified to protect sensitive sources and methods evidences Awlaki's involvement in the planning of numerous other plots against U.S. and Western interests and makes clear he was continuing to plot attacks when he was killed," Holder wrote. He added, "The decision to target Anwar al-Awlaki was lawful, it was considered, and it was just."

Obama announced the death of Al-Awlaki on Sept. 30, 2011, and credited U.S. intelligence agencies, but he did not explicitly acknowledge that Al-Awlaki had been killed by a U.S. strike.

Holder said his letter was only one of a number of steps the administration will be taking to fulfill pledges that Obama has made to provide "Congress and the American people with as much information as possible about our sensitive counterterrorism operations."

To that end, he also disclosed that Obama recently approved "exacting standards and processes for reviewing and approving operations to capture or use lethal force against terrorist targets outside the United States and areas of active hostilities; these standards and processes are either already in place or are to be transitioned into place."

While the procedures will be briefed to select lawmakers, they will remain secret, Holder said, adding that they "make clear that a cornerstone of the administration's policy" is that "lethal force should not be used when it is feasible to capture a terrorist suspect."

Critics were not assuaged by Holder's letter. "The Obama administration continues to claim authority to kill virtually anyone anywhere in the world under the 'global battlefield' legal theory and a radical redefinition of the concept of imminence," said Zeke Johnson, an official with Amnesty International. "President Obama should reject these concepts in his speech tomorrow and commit to upholding human rights, not just in word but in deed."

The McClatchy Washington Bureau contributed to this report.