According to the investigation, which was overseen by Air Force Brig. Gen. Matthew C. Isler, the episode began on that March morning when two Islamic State snipers in the city’s Jidideh section began firing at troops from Iraq’s Counterterrorism Service, which was fighting its way into west Mosul.
General Isler said in a briefing for reporters that his assessment was that the Islamic State deliberately created a situation in which the United States would strike the building and set off the explosions.
The Iraqis did request an airstrike, and the American-led coalition responded by dropping a single GBU-38 munition, which carries about 200 pounds of explosives, around 8:30 a.m.
The aim was to produce a blast that would destroy only the top floor of the building, which was described as a well-built concrete structure, and kill the snipers. The bomb that was dropped, coalition officials said, was not strong enough to have taken down the building.
But the blast set off the explosive material that Islamic State fighters has placed in the structure, causing it to collapse. Analysis of the debris found residue of explosive materials that Islamic State fighters are known to use but that are not used in the GBU-38.
In addition, the investigation said that the main damage to the structure was elsewhere in the building from where the GBU-38 was dropped.
The events that led to the episode, however, are somewhat complicated. According to accounts from neighbors, the civilians went to the building voluntarily because it was one of the best constructed in the area; they took refuge in the basement to get away from the fighting.
The American-led coalition was not aware that civilians had gone there, perhaps because bad weather interfered with reconnaissance in the two days leading up to the strike. Nor did Iraqi forces always have a clear view of the area around the building.
The general said that it was possible that civilians in the building were held against their will after they took sanctuary, though there is no proof that this is the case. Residents of the adjacent building, which was also damaged, he said, were ordered not to leave the night before the attack.
The airstrike was approved by the command center in Erbil. General Isler would not disclose the rank of the officer who approved the strike, but it is usually done by a one-star general.
General Isler said that the American-led coalition had adjusted its tactics and procedures to prevent a repeat of the episode. But he would not detail them, saying the changes were being kept classified so that the Islamic State would not know them.
The American-led command has previously said that 396 civilians have been killed in the more than two-year campaign against the Islamic State in Iraq and Syria, though some independent estimates run much higher.