US marines violated international humanitarian law by using excessive violence in reaction to a suicide bomb attack in Afghanistan, a report says.
The Afghan report said the US marines used indiscriminate force
The reaction was disproportionate and indiscriminate force used, it said.
At least 12 civilians died and 35 were injured during the incident which took place on 4 March in Nangarhar province.
A preliminary US investigation agreed with the report that the unit did not come under small-arms fire after the bombing, US media reports said.
Maj Gen Frank H Kearney III, who ordered the inquiry, told the Washington Post newspaper it had found no evidence that the victims were fighters.
"My investigating officer believes these folks were innocent," he was quoted as saying.
A US military spokesman said shortly after the incident that the civilians might have been killed by incoming fire from an ambush by insurgents which followed the bombing.
The Afghan report said that, in failing to distinguish between civilian and legitimate military targets, the US marine corps used "indiscriminate force".
"Their actions thus constitute a serious violation of international humanitarian law standards," it said.
Evidence of a complex ambush involving militant gunmen who fired on the convoy was "far from conclusive", the report said.
According to the authors of the report, who spoke to victims, police and hospital officials as well as eyewitnesses, the marines fired indiscriminately on civilians and their vehicles as they left the scene.
Meanwhile Maj Gen Kearney said no ammunition casings had been found that might substantiate reports that the marines were fired on.
"We found ... no brass that we can confirm that small-arms fire came at them," he told the Washington Post.
"We have testimony from marines that is in conflict with unanimous testimony from civilians at the site."
Journalists said US soldiers deleted footage, including photos and videos, showing the aftermath of the Nangarhar violence.
The soldiers were moved out of Afghanistan after the incident.