Maine Sens. Susan Collins and Angus King joined other members of Congress on Thursday in calling for a thorough investigation of a U.S. drone strike in January that inadvertently killed two hostages, one of them an American.

The strike on an al-Qaida compound in the border region between Afghanistan and Pakistan killed hostages Warren Weinstein of Maryland and Giovanni Lo Porto of Italy, both international aid workers, as well as an al-Qaida leader who was a U.S. citizen, the government revealed Thursday.

President Obama said he took “full responsibility” for the counterterror missions and offered his “grief and condolences” to the families of the hostages, noting that he has directed a full review of what happened to avoid similar tragedies in the future.

Both Collins, a Republican, and King, an independent, offered condolences as members of the Senate Intelligence Committee but stressed the need to know more.

“I am deeply saddened by today’s announcement and my thoughts and prayers are with the families and friends of Mr. Weinstein and Mr. Lo Porto,” King said in a written statement. “I have been following this closely and I firmly believe that an exhaustive, independent review is necessary to fully understand the circumstances that led to their deaths and to help prevent such terrible tragedies from happening again.”

Collins said Obama was right to take public responsibility for operations against terrorism. However, it’s al-Qaida “that bears the ultimate responsibility for taking these individuals as hostages,” she said.

“I expect that the thorough review promised by the president will be shared with the appropriate committees,” Collins said in a written statement, “but it is important that the Senate Intelligence Committee conduct its own investigation to determine how this tragedy could have occurred.”

Collins said counterterrorism operations must be done with “the utmost care” to avoid casualties beyond intended targets.

Among those believed killed in the drone strike was Ahmed Farouq, who was an al-Qaida leader with dual U.S. and Pakistani citizenship, according to U.S. officials cited in an Associated Press report. U.S. officials also concluded that Adam Gadahn, an American who had served as a spokesman for the terror network, was killed in a separate drone strike in January.

The White House said neither Farouq nor Gadahn was intentionally targeted in the strikes and the United States didn’t have information indicating their presence at the site of the operations.

A U.S. government official told the AP that the drone strikes occurred on Jan. 14 and Jan. 19 in the border region between Afghanistan and Pakistan. The official was not authorized to discuss details of the attacks and spoke on condition of anonymity.

When Obama spoke at the White House on Thursday, he made no mention of Farouq and Gadahn. Instead, he focused his remarks on Weinstein, who had been held by al-Qaida since 2011, and Lo Porto, who had been held since 2012.

Obama defended the legality of the drone strike that killed the hostages and said there had been no evidence that the two men were present at what the United States had determined was an al-Qaida compound.

“Based on the intelligence that we had obtained at the time, including hundreds of hours of surveillance, we believed that this was an al-Qaida compound, that no civilians were present and that capturing these terrorists was not possible,” Obama said at the White House. “And we do believe that the operation did take out dangerous members of al-Qaida.”

Obama expressed regret for the deaths of the two hostages and offered condolences to their families.

“I realize there are no words that can ever equal their loss,” he said.

Weinstein, a 73-year-old development worker from Rockville, Maryland, was abducted in August 2011. His capture came four days before his seven-year stint with the U.S. Agency for International Development was to end.

In a message released previously in English and Arabic and addressed to Weinstein’s family, al-Qaida said it was “not interested in keeping” Weinstein but wanted to exchange him for prisoners in U.S. custody. Lo Porto went missing in Pakistan in January 2012.

Sen. Dianne Feinstein, D-California, vice chairwoman of the intelligence committee, described Weinstein and Lo Porto as selfless role models helping people around the world.

“The committee has already been reviewing the specific January operation that led to these deaths, and I now intend to review that operation in greater detail,” Feinstein said in a written statement. “We should also again review all procedures and safeguards to make sure every measure is taken to prevent the deaths of innocent civilians.”

Rep. John Delaney, D-Maryland, who represented Weinstein in the House, said Weinstein was a former Peace Corps member “who represented the very best of our country.”

“I feel like his country failed him in his greatest time of need,” Delaney said in a written statement. “I’m determined to ensure that Warren’s story is not forgotten, that we get to the bottom of why Warren wasn’t found and how he was killed.”

Delaney said he is developing legislation to streamline and improve the U.S. response to hostage takings.

“It is essential that the review of this tragic accident be a true investigation that focuses on the events surrounding Warren’s death and the intelligence efforts that were deployed to locate him,” Delaney said. “Our national security and intelligence response to hostages must improve and improve quickly.”

Sen. Richard Burr, R-North Carolina, chairman of the intelligence committee, said Thursday’s news was heartbreaking.

“In our grief, though, we should remember that al-Qaida is merciless in their capture and continued detainment of hostages who have selflessly sought to serve others, and they bear the ultimate responsibility for these deaths,” Burr said in a written statement.

“Congress will continue to conduct vigorous oversight of our nation’s counterterrorism operations,” Burr said. “I firmly believe the dedicated Americans engaged in those efforts take very seriously the need to prevent collateral damage and limit the risk to innocent people, and they, too, share in the grief being felt by the Weinstein and Lo Porto families.”

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