Pentagon Blames 105 Civilian Deaths From Mosul Strike On 'Secondary Explosion'

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Updated at 2 p.m. ET

U.S. personnel "could not have predicted" that dozens of Mosul residents would be in a building where ISIS snipers were firing when they authorized a strike on it in March, the Pentagon says in a newly released report. That airstrike in Iraq killed at least 105 civilians.

The report also says the building collapsed after the strike triggered explosives that had been planted by ISIS.

"Our condolences go out to all those that were affected," said Maj. Gen. Joe Martin, commanding general of the ground force fighting ISIS. "The Coalition takes every feasible measure to protect civilians from harm. The best way to protect civilians is to defeat ISIS."

The report provides new details about a strike that devastated several families and prompted rights groups to accuse the U.S.-led coalition of not taking adequate precautions to protect people in Mosul, which Iraq is trying to reclaim from the terrorist group.

It wasn't until after the attack that officials learned civilians were in the targeted building, the Pentagon says. Between 101 and 137 people are believed to have sought refuge there, according to an executive summary of an investigative report prepared for the commander of Operation Inherent Resolve.

Of the 105 civilians who died, 101 died in the structure and another four died from damage to the neighboring building, the Pentagon says.

"Eye witnesses and media outlets reported that an additional 36 civilians may have been at the building," according to the report, "however, there is insufficient evidence to determine their status or whereabouts at this time."

The strike had targeted two ISIS snipers who were on the building's second floor; the U.S. bomb was expected to damage no more than 20 percent of the structure, the Pentagon says. Instead, the building was reduced to a crater of rubble, after ISIS-emplaced explosives detonated, the report says.

The Pentagon says the home that was hit on March 17 either was used to store bombs or was rigged with explosives that "conservatively contained more than four times the net explosive weight" of the 500-pound munition (with 192 pounds of explosive material) that was dropped on the building.

A summary of the report says the munition, described as a "GBU-38 with a delayed fuse," was chosen by the coalition's Target Engagement Authority in the belief that it would achieve "the necessary effect and minimize collateral damage."

Coalition investigators' analysis of the blast site found "residues common to explosives used by ISIS, but not consistent with the explosive content of a GBU-38," the Pentagon says. One of those explosives was nitroglycerin, which investigators say was "likely used by ISIS to sensitize the secondary explosive devices to detonate more easily."

The report adds that the crater at the rear of the structure was in a different area from where the GBU-38 entered and detonated on the second floor.

The civilians who died in the building had been forced out of their houses by ISIS fighters and invited into the home by its owner, the report says. It adds that ISIS fighters had interacted with the residents — and that a next-door neighbor told investigators he had been warned to evacuate his family and move north by the morning of March 17 to avoid danger.

The Pentagon's summary was prepared by the investigating officer, Air Force Brig. Gen. Matthew C. Isler.

After the strike, Iraqi rescue workers told NPR they found dozens of bodies in the rubble. Coming amid a new offensive to try to retake Mosul from ISIS, the attack set off a debate over how to prevent civilian casualties when people are used as human shields — and whether the U.S.-led coalition has been cautious enough in its attacks.

Survivors of the strike told NPR's Jane Arraf about the devastating losses they suffered, and why they hadn't been able to leave.

"Three times we tried to leave and ISIS sent us back," Ala'a Hassan told Jane. "They fired in the air and in the end they said if you try to leave we will hang you."

The deaths prompted Amnesty International to accuse the U.S.-led coalition of not taking adequate precautions to protect people in Mosul, not providing a safe escape route for civilians, and using munitions that are too powerful.

The group's senior crisis response adviser, Donatella Rovera, also acknowledged the challenges in places such as Mosul. Rovera told NPR in March: "There was no easy option because obviously prior to the fighting, Islamic State did not allow people to leave. However, once the fighting got underway, possibilities are created for people to leave."

Within weeks of the strike, as The Two-Way reported, Iraqi and U.S. officials said they would slow the offensive and reduce the number of airstrikes to minimize civilian deaths.

In response to the findings being released Thursday, Amnesty International's Samah Hadid said, "While we welcome the U.S. investigation into the Jadida airstrike, we are curious to know whether any lessons were learned and what steps were taken to ensure such horrors do not occur again."