The White House has said that a US counterterrorism operation in January accidentally killed two hostages who were being held by al-Qaeda.
Warren Weinstein, an American, and Giovanni Lo Porto, an Italian, were killed in the raid in the border region of Afghanistan and Pakistan.
President Barack Obama described it as a painful loss he profoundly regretted.
Two other Americans thought to be al-Qaeda members were also killed, one of them in the same raid.
The White House said Ahmed Farouq, an al-Qaeda leader, was killed in that operation and Adam Gadahn, once regarded as a spokesman for the militant group, was killed in a separate raid.
Unnamed officials told Associated Press the attack that killed the hostages was a CIA drone strike.
Who were the hostages?
- Warren Weinstein, 72, an American development worker, was kidnapped from his home in Lahore, Pakistan, in 2011. He had lived in Pakistan for seven years, working on economic development projects.
- Giovanni Lo Porto, a 39-year-old Italian, was kidnapped in Multan, Pakistan, in January 2012. He worked for an international aid group called Welthungerhilfe
The president, speaking at the White House about the operation that killed the hostages, said the US had launched the raid in the belief the target was an al-Qaeda compound with no civilians present.
As commander-in-chief, he said, he took "full responsibility" for the operation.
The White House said compensation would be paid to the families of the hostages.
Mr Weinstein's wife Elaine said in a statement the family was "devastated".
"Those who took Warren captive over three years ago bear ultimate responsibility," she added.
She thanked several members of the US congress and unnamed officials from the FBI, but said the assistance received from "other elements of the US government was inconsistent and disappointing".
Mrs Weinstein also criticised the Pakistani government and military who, she said, treated her husband's captivity "as more of an annoyance than a priority".
President Obama said the operation was in compliance with the White House's counterterrorism protocols.
It's a "bitter truth in the fog war" that mistakes occur, but what sets America apart is facing up squarely to its mistakes, he said.
Weinstein, 73, was abducted in Lahore in 2011, where he was working as an aid worker. Lo Porto disappeared from Multan, Pakistan in January 2012. Both men were aid workers.
"There could be no starker contrast between these two selfless men and their al-Qaeda captors," the president said.
He also announced a review into the tragedy.
Who was Adam Gadahn?
- Changed his last name from Pearlman, grew up in California and converted to Islam in his teens
- In 1998 moved to Pakistan and worked as a translator for al-Qaeda
- Charged with treason - the first person since the World War II era - in the US and had a $1m bounty on his head
- In 2013 video, called for militants to attack Western diplomats