April 2015, Open Society Justice Initiative: Death by Drone: Civilian Harm Caused by U.S. Targeted Killings in Yemen (PDF)
24 November 2014, Guardian: 41 men targeted but 1,147 people killed: US drone strikes -- the facts on the ground
White House admits: we didn't know who drone strike was aiming to kill
Broad target of 'al-Qaida compounds' suggests 2013 policy change by Obama -- requiring 'near certainty' that suspect is present -- has not been implemented
Spencer Ackerman, Sabrina Siddiqui and Paul Lewis in New York
23 April 2015
The targets of the deadly drone strikes that killed two hostages and two suspected American members of al-Qaida were "al-Qaida compounds" rather than specific terrorist suspects, the White House disclosed on Thursday.
The lack of specificity suggests that despite a much-publicized 2013 policy change  by Barack Obama restricting drone killings by, among other things, requiring "near certainty that the terrorist target is present",  the US continues to launch lethal operations without the necessity of knowing who specifically it seeks to kill, a practice that has come to be known as a "signature strike".
Josh Earnest, the White House spokesman, acknowledged that the January deaths of hostages Warren Weinstein and Giovanni Lo Porto might prompt the tightening of targeting standards ahead of lethal drone and other counter-terrorism strikes. A White House review is under way.
"In the aftermath of a situation like this, it raises legitimate questions about whether additional changes need to be made to these protocols," Earnest said.
Earnest said that "what was targeted" in the two January strikes on the Afghanistan-Pakistan border was "an al-Qaida compound". The two US civilians killed, longtime English-language propagandist Adam Gadahn  and Ahmed Farouq of al-Qaida in the Indian subcontinent, were not "high-value targets" marked for death, he confirmed.
In a May 2013 speech,  Obama indicated that drone strikes were only permissible when the administration possessed "near certainty that no civilians will be killed or injured, the highest standard we can set".
Other criteria unveiled included an actual imminent terrorist threat, even though the Justice Department has held that membership of al-Qaida necessarily implies the posing of such a threat and an absence of viable alternatives to the strikes.
Jameel Jaffer, deputy legal director of the American Civil Liberties Union, said the accidental killings revealed on Thursday raise "questions about the reliability and the depth of the intelligence that the government is relying on to conduct drone strikes".
"In neither of these two cases did the government actually know beforehand who it was killing. It does raise questions about how much the government knows -- or how little the government knows -- before it pulls the trigger," Jaffer said.
"Perhaps that doesn't in itself suggest that the strikes were unlawful, but it certainly raises some questions."
What Earnest described as hundreds of hours of surveillance of the compounds, as well as "near continuous surveillance" in the days ahead of the strike on the Afghanistan-Pakistan border, did not result in information that hostages were inside.
"In an environment like this, absolutely certainty is just not possible," Earnest said of the intelligence gathering efforts along the remote border.
The CIA and the US military's Joint Special Operations Command, both of which carry out drone strikes, have in the past been empowered to launch lethal "signature strikes", which do not require foreknowledge of who specific individuals are before killing them.
Obama's 2013 speech, which avoided confirmation of the signature strikes, seemed to signal that US counter-terrorism efforts from that point on would restrict or abandon the practice.
Yet, noted Micah Zenko of the Council on Foreign Relations, "the administration never said the signature strikes ended in May 2013, on or off the record ... in fact, they never ended."
In addition to the pledged White House review, an "inspector general" is also analyzing the mistaken strikes, although the administration will not identify whether the CIA's inspector general is performing that review.
The leadership of the Senate intelligence committee, which has long supported CIA drone strikes, pledged "vigorous oversight", chairman Richard Burr said. His Democratic counterpart, Dianne Feinstein of California, urged the administration to release an annual report on "both combatant and civilian" deaths from US drone strikes.
Zenko said it was "strange and bizarre" that lethal US operations have received less public scrutiny than CIA torture, the subject of a 6,000-page Senate report  partially released last year.
Human-rights observers see little indication, two years after Obama's speech, that the US meets its own stated standards on preventing civilian casualties in counter-terrorism operations. Reprieve, looking at US drone strikes in Yemen and Pakistan, concluded last year that the US killed nearly 1,150 people while targeting 41 individuals. 
Reprieve lawyer Alka Pradhan, who represents victims of drone strikes, praised the administration for apologizing to family members of Weinstein and Lo Porto, but warned that without increased and consistent transparency it will face charges of hypocrisy from the drone-attack survivors whose plight the administration has never acknowledged.
"The White House is setting a dangerous precedent - that if you are western and hit by accident we'll say we are sorry, but we'll put up a stone wall of silence if you are a Yemeni or Pakistani civilian who lost an innocent loved one. Inconsistencies like this are seen around the world as hypocritical, and do the United States' image real harm," Pradhan said in a statement.
Earlier this month, a collaborative report  from the Open Society Justice Initiative and researchers from the Yemeni nongovernmental organization Mwatana Organization for Human Rights documented nine US air strikes between May 2012 and April 2014 that caused civilian harm.
Based on interviews with victims and their relatives, eyewitnesses, doctors and hospital staff, the report cites a US drone strike that killed 12 people, including a pregnant woman and three children, as well as another incident in which the US struck a house containing 19 people, including women and children.
Earnest said that although the US considered the compound a legitimate target used by al-Qaida, "the operation would not have been carried out" had the US known about the presence of the hostages.