U.S.: More than 100 civilians killed in Iraq bombing in March

A U.S. bomb that dropped on a building in the Iraqi city of Mosul set off explosive materials that had already been placed inside by Islamic State fighters, causing the structure to collapse and killing more than 100 civilians, a U.S. military investigation concluded Thursday.

The 500-pound bomb dropped by a U.S. aircraft on March 17 was intended to kill two IS snipers who posed a threat to Iraqi counterterrorism forces, the lead investigator, U.S. Air Force Brig. Gen. Matthew Isler, said in a telephone briefing for reporters at the Pentagon.

The probe found that the U.S. bomb triggered secondary explosions from devices clandestinely planted in the lower floors of the concrete building, Isler said. He said neither the Iraqi troops nor the Americans who authorized and conducted the airstrike knew civilians were in the building or that the explosive materials were present.

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

Isler said 101 civilians in the building were killed, and another four died in a nearby building. He said 36 civilians remain unaccounted for. The deaths represent about a quarter of all civilian deaths associated with U.S. airstrikes since the air campaign began in 2014.