The Pentagon completed its two-month probe of a U.S. airstrike in Mosul, concluding ISIS militants booby-trapped the building where more than 100 civilians were killed.
"The investigation determined that ISIS emplaced a large amount of explosive material in a structure containing a significant number of civilians and then attacked Iraqi forces from the structure," the Department of Defense said Thursday.
A team of investigators visited the western Mosul site after the March 17 airstrike and detected post-blast residues common from explosives used by ISIS. The probe also used modeling to look at the structural damage to the building and concluded it could not have been caused by the GBU-38 precision-guided munition, or 500-pound bomb, dropped by the U.S. aircraft.
"A secondary explosion triggered a rapid failure of the structure, in which two ISIS snipers, 101 civilians sheltered in the bottom floors of the structure, and four civilians in a neighboring structure were killed," DoD said.
"Neither coalition nor Iraqi forces knew that civilians were sheltered within the structure."
Another 36 civilians remain unaccounted for in the airstrike, which caused the deadliest single incident in the battle to retake Mosul.
The DoD said the coalition selected a single GBU-38 munition because it was "the most appropriate and proportionate means of engagement to neutralize the threat and to minimize collateral damage.
The GBU-38's detonation, localized to the top floor of the structure, ignited a large amount of explosive material which, unknown to the coalition, ISIS fighters had previously placed in the house."