Chip Somodevilla / Chip Somodevilla / Getty Images file Attorney General Eric Holder testifies before the Senate Judiciary Committee on March 6.

The Obama administration publicly acknowledged for the first time Wednesday that U.S. drone strikes have killed four American citizens since 2009, including the previously undisclosed death of a North Carolina resident who left the United States for Pakistan and was later indicted on federal terrorism charges.

Attorney General Eric Holder, in a letter to congressional leaders and chairman of key congressional committees made public on the eve of what was billed as a major counterterrorism speech by President Barack Obama, also confirmed the deaths in drone attacks in Yemen of three other Americans that already had been widely reported: those of radical cleric Anwar Al-Awlaki , his teenage son, Abd al-Rahmn Anwar al-Awlaki; and Samir Khan, the American who ran al Qaeda’s web-based propaganda magazine Inspire. Previously the Obama administration had only acknowledged the senior Awlaki’s killing and refused to publicly confirm or deny reports of the other deaths.

The letter also confirmed that U.S. drones had killed Jude Kenan Mohammed of Raleigh, N.C., more than a year after a local news report quoted a friend as saying he had died in an attack in Pakistan in November 2011.

Holder said in the letter that the senior Awlaki was the only U.S. citizen targeted in a drone strike.

Anonymous / AP Anwar al-Awlaki, a U.S.-born Yemeni cleric and recruiter for al Qaeda in the Arabian Peninsula in Yemen, is shown in an October 2008 file photo.

He also provided new details about what the U.S. says were Awlaki's operational roles in terror plots, including his role in a 2010 attempt to bomb cargo planes by putting bombs in printer cartridges.

It also included an explicit explanation of the U.S. policy for targeted killings of Americans, much of which was included in a “white paper” obtained by NBC News in February.

Mohammed’s death appears to have been news to the FBI, which as of Thursday still listed him on its “most wanted” list, saying, “On July 22, 2009, a federal grand jury in North Carolina indicted Jude Kenan Mohammad for conspiracy to provide material support to terrorists and conspiracy to murder, kidnap, maim, and injure persons in a foreign country. Mohammad is at large … (and) is believed to be in Pakistan.”

A law enforcement official who spoke on condition of anonymity told NBC News: “We don’t know when he was killed. That fact was classified.”

FBI spokeswoman Shelley Lynch said in an email: "Jude Kenan Mohammed remained wanted until there was official confirmation of death. Until now, the matter was classified and it is now appropriate for the wanted poster to be removed from our website."

Obama is expected to discuss the drone program Thursday in a speech at the National Defense University in Washington, D.C.

Release of Holder’s letter came as classified documents obtained by NBC News raised new questions about the CIA-run drone program and whether it is consistent with public comments by Obama and other administration officials describing the strikes as “very precise” and targeted at specific al Qaeda operatives and their associates. In fact, the documents show, the agency has frequently attacked low-level militants and foreign fighters in Pakistan whose names and nationalities were not known, as well as militant groups not directly connected to al Qaeda.

The documents, similar to those recently reported by McClatchy Newspapers, offer a window into the secretive drone program and how its actual operations sometimes differ from the public accounts provided by the administration.

They appear to officially confirm that the agency has engaged in “signature strikes” – a much discussed and controversial practice that has never been publicly acknowledged -- in which CIA drone operators target individuals based on the “signature characteristics” of suspects but whose actual identities are not clear.

They surface at a time that U.S officials appear to be scaling back the drone program – amid warnings from some former military and intelligence officials that the attacks may be creating a backlash harmful to U.S. interests in the long run.

When Obama was asked about the drone program last year during a Google News forum, he called it “a targeted, focused effort at people who are on a list of active terrorists.” In an April 2012 speech, then White House counter-terrorism adviser and now CIA Director John Brennan said: “The United States Government conducts targeted strikes against specific al Qaeda terrorists,” while acknowledging that drone targets included “associated forces.”

But a CIA list of 53 drone strikes in the fall of 2010 indicates that fewer than half – 22 -- listed al Qaeda operatives as the targets. Other strikes were aimed at targets that included suspected members of the militant al-Haqqani network in Pakistan, which is believed to have harbored and worked with al Qaeda; members of the Pakistani Taliban, an Islamic fundamentalist military group that aims to overthrow the Pakistani government; and members of another Pakistani terrorist network identified as the “Commander Nazir Group.” Fourteen of the strikes listed the targets only as “other militants.”

Agency lists for other periods show a higher proportion of strikes being specifically aimed at Al Qaeda operatives. For example, during a nine month period between January and September 2011, 28 out of 42 strikes listed al Qaeda members as targets.

But in other accounts of the strikes, agency officials refer to the targeting of individuals whose identifies do not appear to be known. One 2009 attack was described as being aimed at “military aged males” at a site “associated with al Qaeda explosives training.” Another, in 2010, described the target as “four adult males conducting weapons training.”

The CIA and White House did not respond to requests for comment about the documents. But U.S. officials have vigorously defended the drone program and their public accounts of it, while saying they are limited in what they can say because of its classified nature and the potential impacts of full public disclosure in Pakistan. As for the use of signature strikes , they have argued that “when you have a bunch of guys building explosives, you don’t need to know who they are. They are an imminent threat.”

NBC News’ Pete Williams, Chuck Todd and Tom Curry contributed to this report.

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