Irbil, Iraq
An unclassified summary of the U.S. military investigation into the March 17 incident determined that the 500-pound bomb used in the strike set off additional explosives that were placed in the building by the Islamic State, causing the collapse of the structure.
The blast killed two Islamic State snipers and 105 civilians, including four in an adjacent house in western Mosul’s al Jadidah district, the summary said. Thirty-six additional civilians who were allegedly killed could not be accounted for because of “insufficient evidence to determine their status or whereabouts.” In the days after the strike, some reports said that more than 200 bodies were pulled from the rubble.
The strike is probably the single deadliest civilian casualty incident in the nearly three-year-old air campaign against the Islamic State in Iraq and Syria. Following the public outcry from the strike, the U.S.-led coalition sharply decreased the number of sorties over Mosul in April, according to data analyzed by the nonprofit monitoring group Airwars.
In addition to determining the cause of the building collapse, the investigation also said that the mass of people killed in the strike had been invited to take shelter on the building’s lower floors by a neighbor. The Islamic State also warned them to leave, according to locals interviewed during the investigation.
This account is at odds with what the U.S.-led coalition’s top officer suggested eleven days after the strike during a Pentagon press briefing. At the time, U.S. Army Gen. Stephen Townsend said that it appeared that the civilians were herded there by the Islamic State and were being used as shields.
“What I don’t know is, were they gathered there by the enemy? We still have some assessments to do. I would say this — that it sure looks like they were,” Townsend said on March 28. “The Iraqis firmly believe that they were gathered there by the enemy. And the people we’re talking to say that they were gathered there.”
In reality, U.S. forces and Iraqi troops failed to observe the mass of people enter the building even though the Iraqis on the ground could observe the building for three days before the strike, according to the summary.
On March 17, the Iraqi troops — the U.S.-trained Counter-Terror Services — had planned to advance into the neighborhood. Until that point, U.S. drones and other aircraft had observed Islamic State fighters evicting families in the area and shoring up their defenses in anticipation of the attack, according to the summary.
The request for an airstrike was relayed from Iraqis on the ground to a U.S. targeting cell where it was approved. U.S. forces selected a GPS-guided 500-pound bomb with a delayed fuze to eliminate the snipers.
The bomb, called a GBU-38, was selected because it could penetrate the roof of the building while “minimizing risk to collateral structures,” the summary said. The bomb’s effects were expected to be contained to the second floor of the building, but “the GBU-38 sparked a secondary detonation of a large amount of explosive material that ISIS had previously emplaced in the structure.”