A U.S. military airstrike in March led to the death of more than 100 Iraqi civilians when a dropped bomb inadvertently set off a cache of explosives left in a Mosul building by the Islamic State in Iraq and Syria (ISIS), U.S. Central Command officials revealed Thursday.

The March 17 incident — one of the deadliest civilian casualty events since the beginning of the Iraq War in 2003 — occurred when a U.S. military warplane dropped a GBU-38 precision-guided missile on two ISIS snipers firing on Iraqi forces, Air Force Brig. Gen. Matthew Isler told Pentagon reporters by phone.

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The Iraqis requested the airstrike on the snipers, who were firing from the roof of a building in the densely packed neighborhood of Al Jadidah. U.S.-led coalition forces responded by dropping the 500-pound bomb around 8:30 a.m.

The GBU-38 should have only killed the ISIS snipers and damaged the roof and top floor of the two-story building, which was made with reinforced concrete and described by neighbors as a strong place to seek shelter, Isler said.

But ISIS fighters had rigged the building with roughly 1,000 pounds of secondary explosives “unknown to the coalition.”

The impact from the GBU-38 triggered the ISIS-placed explosives, causing “a rapid failure of the structure.” The collapse killed 101 civilians sheltered in the bottom floor of the building and four in a nearby building. Isler added that 36 civilians remain unaccounted for.

Following the incident, Isler led a team of military investigators in a two-month probe that spoke to more than 20 witnesses and sifted through more than 700 videos taken from coalition warplanes over 10 days before, during and after the airstrike.

An analysis of the building debris reportedly found “explosive residues that are common” in ISIS-made bombs but not found in the GBU-38. It also determined that ISIS had placed “more than four times the net explosive weight in the GBU-38” — which holds about 190 pounds of explosives — in the building.

Isler also stressed that Iraq Counter Terrorism Service (CTS) forces had watched the area around the building for more than two days and had not observed civilians enter or use the structure.

“Neither coalition nor CTS forces knew that civilians were sheltered in the bottom floor of the structure,” he added.

He concluded that “the coalition takes responsibility for the airstrike conducted on March 17” but that “coalition munition was not responsible for the structural failure of the building and the deaths of the civilians inside.”

Critics of the airstrike have said U.S. forces have been too loose with their rules of engagement in the fight against ISIS and have not been careful with the use of air power.

But investigators placed the blame on ISIS.

“This investigation determined that ISIS deliberately stage explosives and snipers to harm civilians,” Isler said. He added that the rules of engagement had not been altered.

The investigation also found that ISIS fighters had not herded civilians into the building prior to the collapse, and that the residents had instead sought shelter in the building. Isler did add that the civilians may not have been allowed to leave in the days leading up to the incident.

“Our condolences go out to all those that were affected,” Combined Joint Forces Land Component Command - Operation Inherent Resolve head Maj. Gen. Joe Martin said in a statement. "The Coalition takes every feasible measure to protect civilians from harm. The best way to protect civilians is to defeat ISIS."

Following the investigation's release, Amnesty International said there was "an urgent need to protect civilians."

"While we welcome the US 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," Amnesty International Middle East Deputy Director of Campaigns Samah Hadid said in a statement.

“We are in no position at this stage to confirm or deny the results of the US investigation," he added. "But as long as the conflict in Iraq is still raging, we call on Iraqi and US-led coalition forces to desist from using explosive weapons with wide area effects, including artillery and mortars in crowded residential areas and to take all feasible precautions to avoid civilian casualties.”