The US government has acknowledged the killing of four American citizens as part of its drone attack program – one person intentionally and three others not specifically targeted but killed in strikes aimed at terrorist suspects.

The information comes in a letter to congressional leaders from Attorney General Eric Holder, reported Wednesday by several news organizations, first by The New York Times.

The news comes on the eve of a major speech President Obama is scheduled to give on national security issues at the National Defense University on Thursday, and it focuses on what could be the most controversial subject Obama intends to cover.

The one US citizen targeted was radical Muslim cleric Anwar al-Awlaki, killed in Yemen in 2011. Also killed in strikes aimed at other individuals were Mr. Awlaki’s 16-year-old son Abdulrahman al-Awlaki, Samir Khan (both killed in Yemen), and Jude Mohammed, who was killed in a strike in Pakistan.

Mr. Kahn was the publisher of Inspire magazine, which Mr. al-Awlaki edited, ABC News reports. Mr. Mohammed was on the FBI's 10 Most Wanted List and was believed to be plotting a car bombing on the 10th anniversary of 9/11.

“These individuals were not specifically targeted by the United States,” Mr. Holder acknowledges without citing further information on those three US citizens killed in drone attacks.

The bulk of Holder’s letter reiterates the legal justification for killing Americans abroad laid out in a speech the attorney general made in March at Northwestern University Law School, as well as the specific justification for killing al-Awlaki.

"Based on generations-old legal principles and Supreme Court decisions handed down from WWII, as well as during the current conflict, it is clear and logical the United States Citizenship alone does not make such citizens immune from being targeted," Holder wrote in his letter this week.

The decision to deliberately kill al-Awlaki underwent "exceptionally rigorous" review, Holder wrote, and was approved only because he was "a senior operational leader" within Al Qaeda. "The decision to target Anwar al-Awlaki was lawful, it was considered, and it was just.”

Holder described al-Awlaki as “a senior operational leader of Al Qaeda in the Arabian Peninsula (AQAP), the most dangerous regional affiliate of Al Qaeda and a group that has committed numerous terrorist attacks overseas and attempted multiple times to conduct terrorist attacks against the US homeland…. He was the group’s chief of external operations, intimately involved in detailed planning and putting in place plots against US persons.”

“In this role,” Holder continued, “al-Awlaki repeatedly made clear his intent to attack US persons and his hope that these attacks would take American lives.”

Specifically, US intelligence directly connected al-Awlaki to the “underwear bomber’s” attempt to blow up an airliner en route to Detroit on Christmas Day 2009 and to the attempt to detonate explosive devices on two US-bound cargo planes in October 2010.

"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.

The drone program under Obama has vastly increased since it began in the Bush administration, and it has come under fire across the political spectrum for its secretiveness as well as for what critics say is a large number of civilian casualties, including children.

The administration was forced to respond last month when US Sen. Rand Paul (R) of Kentucky filibustered for 13 hours in protest of a policy which allows for the drone targeting of US citizens affiliated with terrorist groups.

Critics were not assuaged by Holder’s letter, The New York Times reports. “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.”

In his speech Thursday, Obama is expected to address another controversial subject: the US military prison at Guantánamo Bay, Cuba.

When he first ran for the White House in 2008, Obama pledged to close Guantánamo Bay, but congressional opponents have been able to block that ever since.

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"It is expensive. It is inefficient. It hurts us in terms of our international standing. It lessens cooperation with our allies on counterterrorism efforts. It is a recruitment for extremists. It needs to be closed," the president said recently.

As of Sunday, The Guardian newspaper reports, 103 of the 166 inmates at Guantánamo Bay are on a hunger strike that began about three months ago, and 30 of those inmates are being force-fed.