Prior to the March 17 strike, Iraqi forces were about 100 meters (328 feet) away and could see two snipers on the second floor of the building. A US aircraft carried out that strike with a single GBU-38 Joint Direct Attack Munition after receiving approval from the Coalition Target Engagement Authority in Erbil, Iraq.
Thirty-six civilians believed to have been inside the building have not been found.
According to the Pentagon's report, the GBU-38 bomb should have killed the snipers but left the two-storey building, which was made of reinforced concrete with 30in walls, intact.
The probe found that the USA bomb triggered secondary explosions from devices clandestinely planted there by ISIS fighters.
The structural failure of the compound and cratering was not the result of a single GBU-38, experts contend, but the result of ISIS placing explosives, four times as much as the explosive material found in the USA precision-guided munition, near the rear of the building's second story. Isler said IS militants had easy access to the second floor and could have quickly planted explosives out of sight of others.
And that's exactly what the al-Jadida district was on the west side of Mosul, which is where all of this happened.
The US military said it makes every effort to minimize civilian casualties, even forgoing possible strikes on ISIS if it could potentially lead to civilian casualties. The expelled families, attempting to seek shelter, were welcomed into the large compound by a neighbor.
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Mosul resident Khalid Zakhi, who survived the attack but whose wife and son were killed, said he was encouraged by the report. The Iraqi forces were close enough to have the structure under visual surveillance, Isler said. Iraqi civil defense forces initially put the figure at 278 but scaled that back to 142 on Thursday, after the US report was released.
However, the western part of Mosul, with its narrow streets and heavily populated neighborhoods, appears to be a bigger challenge to the Iraqi forces.
The photographer said he had initially meant to document the heroism of Iraqi forces fighting Islamic State but that a darker side of the war had gradually been revealed to him.
The airstrike detonated the explosives in the building, leading to its collapse. It also found that USA and Iraqi forces were unaware that ISIS had rigged the building with additional explosives.
The U.S. military will make "solatia" or condolence payments if claims from next of kin can be substantiated, one official said.
Human rights groups have welcomed the the USA investigation, with reservations. Another Britain-based group, the Syrian Observatory for Human Rights, on Tuesday said US-led air strikes killed 225 civilians in Syria over the past month.
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.
"No one saw [ISIS] move explosives into the area".
CTS and coalition forces did not know civilians were in the building, Isler said, and ultimately a strike was called in.