US airstrike in Mosul caused explosion that killed more than 100 Iraqi civilians The Pentagon investigated a U.S. airstrike targeting ISIS fighters in March.

 -- A U.S. military investigation has found that 105 civilians were killed in west Mosul in March after a coalition airstrike caused a secondary explosion that was fatal.

Investigators said they believe that ISIS deliberately planted two snipers on the structure's rooftop and rigged the house with explosives to draw a coalition airstrike that would kill the civilians sheltering inside. Neither Iraqi nor coalition forces knew that civilians were inside the home prior to the coalition airstrike.

"This investigation determined that ISIS intentionally staged explosives in a home it knew to be occupied by more than 100 civilians and used this structure as a fighting position to engage Counter Terrorism forces (CTS)" said Brigadier General Matthew Isler, at a Pentagon briefing.

"They put a lot of work into this setup," Isler, who lead the U.S. military's investigation into the deadly airstrike, added.

A secondary explosion triggered by the U.S. airstrike killed 101 civilians inside the home and four others in a neighboring structure. An additional 36 civilians are still missing and may have escaped the airstrike

ISIS had discovered the large number of civilians gathered at the two story home in the Al-Jadidah district in west Mosul, according to Isler. Investigators determined the home was owned by a well-respected village elder and about 140 civilians were believed to have sought refuge there in the days before the strike.

After ISIS "interacted" with the civilians, Isler continued, they rigged the back of the house with explosives and positioned two snipers to draw fire on the house.

Elite CTS forces had taken ISIS sniper fire from the house for days and observed it for two days before they called in an airstrike to eliminate the two ISIS snipers.

While there had been overhead drone surveillance of the neighborhood before the Iraqi troops moved, bad weather prevented overhead surveillance on March 15 and 16.

On March 17, a U.S. aircraft dropped a GBU-38 precision bomb carrying 192 pounds of explosive power that was intended to eliminate the two snipers on the rooftop. That amount of explosives was not enough to bring down the structure, but it triggered a secondary blast from the explosives planted by ISIS.

Though the airstrike had gone through the coalition's vigorous vetting to ensure there were no coalition casualties, Isler said, "Neither Coalition nor CTS forces knew that civilians were sheltered in the bottom floors of the structure."

"Post blast analysis conducted by Coalition Explosive Ordnance Disposal teams' detected explosive residues that are common to ISIS explosives, but are not consistent with the explosive content of the GBU-38," said Isler. "ISIS-emplaced explosive material conservatively contained more than four times the net explosive weight of the GBU-38."

Isler emphasized that the coalition takes responsibility for the airstrike.

In the wake of the incident, the coalition has already adapted new drone tactics to identify civilians being used by ISIS as human shields.

The coalition is reviewing a recommendation by investigators to establish a dedicated team that would work with Iraqi Civil Defense Force to help assess allegations of civilian casualties.

"The creation of this team will help speed-up the assessment process, provide for increased accountability for strikes that may result in civilian casualties, and help capture lessons learned so we can continue to improve our targeting procedures to defeat ISIS, while preserving civilian life and infrastructure to the maximum extent possible," said Isler.