A Pentagon investigation has found that more than 100 civilians were killed after the U.S. dropped a bomb on a building in Mosul, Iraq, in March.

The probe found that the U.S. bomb triggered secondary explosions from devices clandestinely planted there by ISIS fighters. And the military says the secondary blasts caused the concrete building to collapse.

It was likely the largest single incident of civilian deaths since the U.S. air campaign against ISIS began in 2014.

Air Force Brig. Gen. Matthew Isler, the lead investigator, said 101 civilians in the building were killed, and another four died in a nearby building. He says 36 civilians remain unaccounted for.

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The deaths represent about a quarter of all civilian deaths since the U.S. air campaign began.

"Our condolences go out to all those that were affected," said Maj. Gen. Joe Martin, commander of ground forces for the U.S.-led coalition fighting ISIS. "The Coalition takes every feasible measure to protect civilians from harm. The best way to protect civilians is to defeat ISIS."

Residents carry the bodies of several people killed during fighting between Iraq security forces and Islamic State militants on the western side of Mosul, Iraq on March 24, 2017. A U.S. airstrike allegedly killed more than 100 people the previous week. Felipe Dana / AP file

After initially denying that the U.S. conducted any airstrikes in the Mosul al-Jadida neighborhood, about one week later CENTCOM put out a statement acknowledging that they had struck ISIS fighters and equipment in the area at the request of Iraqi forces. The U.S. military then initiated a formal investigation.

Local residents reported that upwards of 200 people were killed in the strike, but the U.S. military has confirmed that between 137 and 140 civilians were likely present at the time of the strike. Even the lower estimates of civilian deaths will likely make this the deadliest U.S. airstrike against civilians since they began flying in Iraq again in 2014 — possibly even since 2003.

The investigation has been complete for days now, according to three U.S. military officials.

The officials said the findings include:

Iraqi Counter Terror Services troops were watching the building for two days prior to the airstrike but overhead surveillance drones were unable to gather useful full-motion video because the weather was bad and clouds obstructed views. The Iraqis did not see any civilians coming and going for the 48 hours prior to the strike, but they were only watching from one vantage point and couldn’t see all entrances to the building.

The U.S. responded to a request for air support from the Iraqis who were taking fire from two snipers in the building. The request for an airstrike was vetted through the proper channels and the U.S. struck the snipers with a GBU-38 joint direct attack munition with a delayed fuse (a 500 lb bomb).The bomb took out the exact area the snipers were firing from on the second floor in the front side of the structure. It was not powerful enough to bring down the reinforced concrete structure.

There was a secondary explosion in the back side of the building that caused the building to collapse. Expert analysis and modeling found that the explosives needed to bring down the building would be more than 4 times what was delivered by the GBU-38.

The U.S. did strike the building and civilians were among the dead. The building was leveled because of explosives that ISIS had put inside. The investigation could not determine whether the civilians were being held hostage in the building nor could it determine definitively whether the building was a storage facility for ISIS explosives or whether it was specifically rigged to collapse.

The building was in a densely populated area in western Mosul.

A few weeks after the strike, the commander of the U.S.-led coalition in Iraq and Syria said that the U.S. probably had a hand in the civilian casualties, but added that ISIS may have gathered civilians there and planted explosives to cause the building to collapse. Lt. Gen Townsend added that the munition used to strike the building was not designed to level it, and that secondary explosions must have been involved.

The U.S. military will make "solatia" or condolence payments if claims from next of kin can be substantiated, one official said. These claims have rarely been substantiated in Iraq or Syria during this current conflict.