America's shadowy drone campaign was thrust under the spotlight today after President Barack Obama admitted that a botched strike had accidentally killed an American and an Italian hostage held by al-Qaeda.
Mr Obama made an emotional statement to offer his "grief and condolences" to the families of Warren Weinstein and Giovanni Lo Porto, who were both killed in a drone strike in Pakistan in January.
The US also said it had killed two American citizens working for al-Qaeda but that neither of the men were deliberately targeted and the CIA did not realise they would die in the strikes.
The admissions raise fresh questions about America's use of drones and how much intelligence the CIA gathers on a potential target before making the decision to strike.
The deaths of two Western hostages prompted Mr Obama to make a rare public statement about classified US operations against al-Qaeda.
"It is a cruel and bitter truth that in the fog of war generally and our fight terrorists specifically, mistakes - sometimes deadly mistakes - can occur," Mr Obama said.
He said he took "full responsibility" for the operation that killed Dr Weinstein and Mr Lo Porto and promised a full review of the mission. Mr Obama did not personally authorise the strike but it was carried out under guidelines he set.
"I profoundly regret what happened," Mr Obama said. "On behalf of the United States government, I offer our deepest apologies to the families."
Both men were killed three months ago but the White House said it had taken time to confirm that they were killed by an American operation.
Dr Weinstein was a Jewish academic and development contractor who was kidnapped in Lahore in August 2011 in the final days of a four-year posting in Pakistan.
Al-Qaeda released several videos touting his Jewish faith and making him plead directly to Mr Obama for his release.
Elaine Weinstein, Dr Weinstein's widow, said she was "devastated by this news and the knowledge that my husband will never safely return home".
While Mrs Weinstein said she had received help from members of Congress and the FBI, she said "the assistance we received from other elements of the US government was inconsistent and disappointing".
Mrs Weinstein said she wanted to know more about the circumstances in which her husband was killed but that al-Qaeda bears "ultimate responsibility" for his death.
Mr Lo Porto studied at London Metropolitan University before moving to Pakistan in early 2012, where he was almost immediately kidnapped along with a German colleague. He spent time on aid projects in the Central African Republic and Haiti before heading to Pakistan.
"Amid grief that is unimaginable, I pray that these two families will find some small measure of solace in knowing that Warren and Giovanni's legacy will endure," Mr Obama said.
Ahmed Farouq, an American al-Qaeda operative, was killed in the same strike that killed Dr Weinstein and Mr Lo Porto.
The US said a separate operation had killed Adam Gadahn, a Californian who was raised in a Jewish family before embracing radical Islam and eventually joining al-Qaeda.
He became a prominent spokesman for Osama bin Laden and would taunt the US in English in a clear American accent.
The US has used drones to target American citizens before, most notably killing the radical cleric Anwar al-Awlaki in Yemen in 2011.
But neither Gadahn nor Farouq were deliberately targeted and the US only learned after the strike that it had killed two of its own citizens working for al-Qaeda.
Drone strikes usually begin with an intelligence lead pointing to a specific target, be it a building, encampment or a vehicle. Surveillance drones then circle above the target for hours or weeks to develop a "pattern of life" and study the movements of people on the ground.
The White House has never published the guidelines it uses before launching a strike but in a major 2013 speech, Mr Obama said the US would only attack if the target represented a "continuing and imminent threat" to the US and there was "near certainty" that no civilians would be killed.
Mr Obama said that the US believed there were "no civilians present" when it launched the January attack that killed the two hostages.
Despite the restrictions, civilians are regularly killed in US strikes in Pakistan, Yemen and Somalia.
In one of the most notorious incidents, al-Awlaki's teenage son, who was an American citizen, was killed by mistake in a 2011 strike in Yemen.
The American Civil Liberties Union, a civil rights groups, said the botched January strikes were evidence the US "quite literally didn't know who it was killing".
"These and other recent strikes in which civilians were killed make clear that there is a significant gap between the relatively stringent standards the government says it’s using and the standards that are actually being used," the group said.
Jonathan Horowitz, an expert with the New York-based Open Society Justice Initiative that recently published a damning report on the extent of civilian casualties in drone strikes in Yemen, said the Obama administration’s admission demanded greater transparency.
“This episode demands the administration examine the consequences of drone programme and put front and centre for US public why they should have as much confidence in the drone programme as the Mr Obama says they should,” he said.
The efficacy of the US lethal drone programme, which began in June 2004, has come under mounting scrutiny in recent years following several high-profile cases where civilian casualties were killed, creating deep resentment among local populations.
Some experts have questioned the utility of the Obama administration’s “decapitation strategy”, arguing that it only serves to splinter and further radicalise terror organisations.
Recently published research by Max Abrahms, a political science professor at Boston’s Northeastern University, has suggested that terror groups with fragmented leaderships are more likely to target civilians.
“My research suggests that drone strikes tend to change the tactical thinking of terror groups and make them more unrestrained and less selective, as well as creating a backlash among otherwise law-abiding local populations who then move to support the very terror groups we are trying to undermine,” he said.