This video showing Warren Weinstein, a U.S. contractor held by al-Qaeda militants, was released in 2013. The full 13-minute video was sent anonymously by e-mail to several journalists who have reported from Afghanistan. (As-Sahab/AFP)

A CIA drone strike in January that was aimed at a suspected al-Qaeda compound in Pakistan accidentally killed two hostages, including a kidnapped American, U.S. officials acknowledged Thursday.

U.S. officials said they did not realize until weeks later that two civilians had died in the attack — kidnapped aid workers Warren Weinstein of Maryland and Giovanni Lo Porto of Italy — despite assurances from the CIA at the time of the operation that only al-Qaeda fighters were present.

The CIA had been conducting surveillance on the site near the Afghan border for hundreds of hours, U.S. officials said.

But the spy agency later discovered the strike had also killed a second U.S. citizen, Ahmed Farouq, who U.S. officials said had joined al-Qaeda years earlier and was among the suspected militants at the compound.

After the CIA slowly pieced together what had happened, the spy agency’s director, John Brennan, delivered the news to President Obama last week. On Thursday, in brief remarks from the White House, a grim and downcast Obama informed the nation of the botched operation.

“As president and as commander in chief, I take full responsibility for all our counterterrorism operations, including the one that inadvertently took the lives of Warren and Giovanni,” Obama said. “I profoundly regret what happened. On behalf of the United States government, I offer our deepest apologies to the families.”

Weinstein, 73, had been held since 2011 after being kidnapped in Lahore, Pakistan. Lo Porto, 39. had been in al-Qaeda captivity since 2012.

Obama said he spoke Wednesday with Weinstein’s wife, Elaine, and Italian Prime Minister Matteo Renzi to inform them.

In a statement, Elaine Weinstein said Thursday that she and her family were “devastated” by the news and “do not yet fully understand all of the facts surrounding Warren’s death.”

“We were so hopeful that those in the U.S. and Pakistani governments with the power to take action and secure his release would have done everything possible to do so and there are no words to do justice to the disappointment and heartbreak we are going through,” she said.

Earnest said the families of the two hostages will receive U.S. government compensation, but he declined to provide details.

A U.S. government contractor kidnapped by al-Qaeda militants in Pakistan in 2011 called on the Obama administration to negotiate with his captors and says he feels “totally abandoned and forgotten.”

Obama said that the operation was “fully consistent with the guidelines” he has established for counterterrorism strikes against al-Qaeda but that he has ordered “a full review of what happened.”

“It is a cruel and bitter truth that in the fog of war generally and our fight against terrorists specifically, mistakes, sometimes deadly mistakes, can occur,” the president added. “But one of the things that sets America apart from many other nations, one of the things that makes us exceptional, is our willingness to confront squarely our imperfections and to learn from our mistakes.”

Obama provided only limited details about the operation. He did not specify how or where the hostages were killed, or which arm of the U.S. government was responsible.

A CIA spokesman declined to comment.

Two Pakistani intelligence officials, both of whom spoke on the condition of anonymity, said they believe Weinstein, Lo Porto and Farouq were killed during a Jan. 15 drone strike in the Shawal Valley in North Waziristan, part of Pakistan’s tribal belt.

A statement released earlier this month by al-Qaeda’s media arm also reported that Farouq had been killed on Jan. 15 in the Shawal Valley, but it did not identify the little-known figure as an American or make any mention of the hostages.

The CIA has been conducting drone strikes against al-Qaeda targets in Pakistan for more than a decade under a covert program first authorized by President George W. Bush and substantially expanded by Obama. The strikes have caused widespread public anger in Pakistan for inflicting civilian casualties but have been tolerated by the Pakistani government as part of an unspoken arrangement with the U.S. government.

Although Obama did not mention it in his remarks, another American was killed in a separate counterterrorism operation in January, the White House acknowledged in a statement Thursday.

Adam Gadahn, 36, a California native who converted to Islam and joined al-Qaeda more than a decade ago, was killed in a CIA drone attack in Pakistan within a week of the strike that killed the hostages, U.S. officials said.

Gadahn, who called himself “Azzam the American” and helped run al-Qaeda’s propaganda department, was indicted by a federal grand jury in 2006 on charges of treason.

As with the strike that killed Farouq and the hostages, U.S. officials said they were targeting a suspected al-Qaeda compound and did not realize that an American citizen was there.

White House press secretary Josh Earnest said Obama had not personally approved the operations but that U.S. counterterrorism officials had the authority to conduct them.

Earnest described Gadahn and Farouq as al-Qaeda leaders but said the U.S. government had not classified either man as a “high-value target,” meaning they were not considered an imminent threat and otherwise would not have been singled out for a lethal attack.

Al-Qaeda had listed Farouq as a leader of its branch in the Indian subcontinent. U.S. officials said he was born in the United States and moved to Pakistan as a child.

It is not the first time that the U.S. government has killed Americans in drone strikes overseas. In 2011, a CIA drone in Yemen targeted and blew up Anwar al-Awlaki, a New Mexico-born cleric who was a key figure in al-Qaeda’s franchise on the Arabian Peninsula.

Four other Americans, including Awlaki’s teenage son, have died in drone attacks. In each of those cases, however, U.S. officials said they were unaware of the Americans’ presence beforehand and described them as incidental casualties.

In December, a failed rescue attempt carried out by U.S. Special Operations forces inadvertently led to the death of Luke Somers, an American held hostage in Yemen.

Thursday’s disclosure of the accidental deaths was sure to bring increased pressure on Obama to curtail or scale back drone strikes, a signature tactic of his presidency.

The bungled operation will also force the White House to confront lingering questions about its policies for responding to the kidnapping of Americans by extremist groups in the Middle East and South Asia.

“I’m saddened, disappointed and outraged that our government was not able to bring Warren home,” said Rep. John Delaney (D-Md.), Weinstein’s representative in Congress. “Today’s news is a personal tragedy for Warren’s family but also a sobering national security and government failure.”

Although Obama has insisted that the CIA and U.S. military take every precaution to avoid civilian casualties, drone strikes have resulted in numerous deaths of Pakistani, Afghan and Yemeni civilians.

Sen. Dianne Feinstein (Calif.), the top Democrat on the Senate Intelligence Committee, said the panel had already been secretly reviewing the January strike that killed Weinstein, Lo Porto and Farouq but would now “review that operation in greater detail.”

Feinstein added that more information should be made public about U.S. counterterrorism strikes, including an annual report on the number of combatants and civilians who are killed.

Jameel Jaffer, deputy legal director at the American Civil Liberties Union, said the January attacks in Pakistan raised doubts about the reliability of the intelligence used to justify drone strikes.

In both operations, he said, “the U.S. 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.”

U.S. officials confirmed that the strike that killed the hostages was a “signature strike” — a category in which the CIA has authority to attack based on suspicious patterns of activity even when it cannot identify the individuals being targeted.

The sequence suggests the hostages had been held at the compound over a long period. Current and former U.S. officials said that analysts watching drone footage can typically detect the movement of hostages by al-Qaeda captors.

It’s not clear how CIA drones, presumably equipped with infrared sensors, would have failed to recognize the presence of two additional people at the compound before it was hit. But officials said al-Qaeda has adapted to the drone campaign by taking extensive measures to obscure its facilities from drone cameras.

The number of U.S. drone strikes in Pakistan has gradually declined since reaching a peak in 2010, when there was, on average, one attack every three days. This year, there have been seven reported drone strikes in Pakistan, but only two since January.

Although Obama said such counterterrorism operations were under review, other U.S. officials said the CIA drone program has not been suspended.

Tim Craig in Mingora, Pakistan, Aamir Iqbal in Peshawar, Pakistan, and Julie Tate, Karen DeYoung, Dan LaMothe and Dan Morse in Washington contributed to this report.

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