A U.S. military investigation has found that a March airstrike in Syria that rights groups had said killed dozens of civilians and partially destroyed a mosque was legal and caused only one civilian casualty.

U.S. Army Brig. Gen. Paul Bontrager told reporters Wednesday that the investigation found no evidence of negligence by the Special Operations Task Force that ordered the March 16 strike in al-Jinah village in Syria’s Aleppo province.

“This investigation found that the strike complied with operational and legal requirements,” Bontrager said. “We are confident this was a meeting of al-Qaeda members and leaders; this was not a meeting of civilians.”

Bontrager said that the investigation into the attack did not involve any interviews with people on the ground and added that no U.S. personnel visited the site.

An April Human Rights Watch report that draws on architecture analysis — including the original plans for the targeted building — and four eyewitness statements said the attack struck the northern part of the Omar Ibn Al-Khatab Mosque, specifically an area with a small prayer room, washroom and living quarters for the imam. That section was connected to a large praying area.

While Bontrager said the U.S. investigation took the Human Rights Watch report into account, he said the targeted structure was under construction and was not a mosque. Bontrager did say, however, that the structure was planned to be a religious school.

Bontrager said that dozens of al-Qaeda fighters were killed while the Human Rights Watch report says that 38 civilians were killed. Twenty-eight were identified as civilians, and 10 were unaccounted for. The U.K.-based Syrian Observatory for Human Rights said at least 49 people were killed in the strike.

Bontrager said that the U.S. investigation “considered media reports that indicated a large number of civilians were killed, but our investigation did not uncover evidence to support those claims.” He added that U.S. forces observed one civilian who was “small in stature” accompany another person into the building. Bontrager stopped short of calling the individual a child.

“This is a mosque. It was a time for prayers. Who do you think was there? People who come to pray. Older people, clerics, children. Children were inside. We were all civilians, there weren’t even people there from the Free Syrian Army,” one eyewitness is quoted as saying in the Human Rights Watch report.

The strike involved two Hellfire missiles and 10 bombs, probably GPS-guided 500-pound variants, dropped from an F-15 fighter aircraft. Bontrager said that U.S. forces received intelligence in the days leading up to the strike that a meeting of al-Qaeda fighters was about to take place in the building.

Bontrager said that none of the buildings in the vicinity of the attack were on the region’s “No-Strike List” — structures such as hospitals, schools and mosques that require higher authorization to be targeted under the Law of Armed Conflict.

“Any structure at all, if it’s being used for a military purpose, can be struck, can be a legal target to strike,” Bontrager said, admitting that the personnel responsible for the strike did not have “all the best information” at the time of the attack. Some members of the unit who participated in the targeting process, however, were aware that the targeted structure had some planned religious purpose, but that information did not make it up the chain of command before the building was struck, Bontrager added.

Bontrager said some of the lessons learned from the investigation involved issues that arose during shift changes at the unit in charge of coordinating the strike. He said that information was not properly relayed between two shifts, a similar charge raised by an investigation into a September strike where U.S. forces accidentally targeted and killed more than a dozen Syrian government troops.

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