US air strikes probably played a role in the death of dozens of civilians in Mosul earlier this month, but an ongoing investigation may reveal a more complicated explanation, the top commander of American forces in Iraq has said.
One possibility is that Islamic State militants rigged the building with explosives.
Speaking from Baghdad to reporters at the Pentagon, Army Lieutenant General Stephen Townsend said a recent spate of civilian casualties in Mosul was "fairly predictable" given the densely populated urban neighbourhoods that Islamic State fighters are defending against Iraqi government troops.
The civilian deaths cannot be attributed to any loosening of American military rules of combat, he said, and Washington has not decided to tolerate greater risk of civilian casualties in US air strikes.
Witnesses say the March 17 explosions may have killed at least 100 people. And Amnesty International on Tuesday said the rising death toll suggested the US-led coalition is not taking adequate precautions as it helps Iraqi forces try to retake the city. The Pentagon arranged a short-notice briefing by Lt Gen Townsend amid the growing chorus of criticism.
Defending US precautions against civilian deaths, Lt Gen Townsend acknowledged the US conducted multiple air strikes in the area of the explosions. Coupled with initial inquiries done by US technical experts who later visited the scene, he said: "My initial assessment is that we probably had a role in these casualties."
But Lt Gen Townsend said the type of munitions used by the US in the air strikes should not have been able to bring down the entire building, raising questions about the level of the American involvement.
US officials also are assessing the possibility that IS forced civilians to gather there to act as human shields or to lure the US into attacking, he added.
"It sure looks like they were," Lt Gen Townsend said. Another possibility he said was being examined: That IS filled the building with explosives.
In the most extensive US explanation of what is known about the event, Lt Gen Townsend stressed that no one should think it was a deliberate US act.
"If we did it - and I'd say there is at least a fair chance we did - it was an unintentional accident of war," he said.
The fight for western Mosul began in December after Iraqi security forces pushed IS out of the eastern side of the Tigris River city. In recent weeks, IS defenders have packed into neighbourhoods with narrow streets and trapped civilians, Lt Gen Townsend said.
"It is there that the enemy has invested two-and-a-half years of defensive preparations," he said. "It is there that the fighting has gotten extraordinarily brutal." He called it the "toughest phase" of the war.
"I think that's really the explanation for the civilian casualties," Lt Gen Townsend added. "Civilians are there. Some of them have been able to escape. Those that have not been able to escape are held against their will" or are afraid to try to leave.
"Although our partners and the coalition have made mistakes that harmed civilians, we have never targeted them - not once," he said.
Lt Gen Townsend also said he believes US air strikes are not to blame in a separate instance of alleged civilian casualties, in Syria. A leading Syrian opposition group, the Syrian National Coalition, has said it believes coalition forces were behind an air strike on March 21 alleged to have killed at least 30 civilians in a school outside of Raqqa, Syria.
"I think that was a clean strike," Lt Gen Townsend said, adding that the allegation that those targeted in the building were civilians is "going to play out to be unfounded".