An internal US military report released Thursday concluded that a March 17 bombing targeting two ISIS snipers in the Iraqi city of Mosul killed at least 105 civilians, marking the largest number of Iraqis US officials have acknowledged killing since its war on the militant group began nearly three years ago.

Another 36 civilians are missing in the aftermath of the strike, the US military said during a briefing to reporters on the investigation’s results.

The incident began around 8 am while Iraqi forces were trying to clear part of the al-Jadidah neighborhood in Mosul, Iraq’s second largest city, as part of the ongoing battle to wrestle the city from ISIS, when those forces came under sniper fire from a nearby two-story building. In response, US dropped a 500-pound bomb on the building in an attempt to kill the snipers.

The US military believes that ISIS rigged the building, which was serving as a shelter for 140 people seeking protection, with a 1,000-pound explosive. That explosive triggered the collapse of the building. US officials the residue of the explosive found was different than that of the bomb, leading them to conclude a second explosive was at the scene. In addition, witnesses reported a second, bigger explosion immediately after the strike.

The US military did not know about the civilians sheltered in the building,nor was it aware that the building had been rigged with explosives when it conducted the strike, Air Force Gen. Matthew Isler, who was part of the investigation, told reporters during a briefing Thursday.

But Isler said that the US could not say when ISIS planted the explosive in the building. He noted that in the two days before the strike, cloud cover made it difficult for US drones to capture videos around the area.

“No one saw [ISIS] move explosives into the area. However, there were multiple opportunities” to do so relatively undetected, Isler explained.

How the US-led coalition mitigates the risk of civilian casualties has been a looming question in its war against ISIS, particularly as it is fighting it largely from the air, where it cannot always accurately assess the effects of its strikes.

US officials have been repeatedly criticized for inadequately calculating the number of civilians killed by its strikes. The densely-populated city of Mosul is particularly susceptible to civilian casualties as the Iraqi forces, backed by US-led coalition strikes, conduct a block-by-block battle against ISIS for control of the city.

The US military said it makes every effort to minimize civilian casualties, even forgoing possible strikes on ISIS if it could potentially lead to civilian casualties. The military will now conduct an on-the-ground assessment of its strikes in Mosul to determine if there were other instances of civilian casualties it was not aware of, defense officials said.

Often coalition strikes happen in areas that are too remote or dangerous for independent observers, troops or even eyewitness can make an on-the-ground assessment. But the March 17 strike was one of the few where independent observers could get to the scene and make firsthand assessment of the damage done by the strike.

Days after the strike, a Kurdish news agency reported that scores of bodies were being pulled out of a building. The US military announced shortly afterward it was investigating report that it had caused civilian casualties.

Before Thursday’s report, the US had concluded it had killed 352 civilians in Iraq and Syria since 2014. Some independent observers say the figure is tenfold.

The US said it has yet to pay out condolence payments to the victims but that a process for such payments exists.