The board of inquiry found that "while the incident was caused by an air attack, it was not possible to identify the perpetrator or perpetrators," said a summary of the findings released yesterday.
The convoy was "subject to an attack from the air, using multiple types of munitions deployed from more than one aircraft and aircraft type," the inquiry found.
The munitions used during the 30-minute assault may have included missiles, rockets and small bombs, it added.
The board rejected allegations that the attack could have been carried out by direct fire or a ground assault in the rebel-held area.
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It noted that Syria and Russia as well as the US-led coalition "all had the capabilities needed to carry out an attack of the kind" that took place that day.
But it concluded the involvement of coalition aircraft was "highly unlikely."
"However, the board did not have access to raw data to support these assertions and, in their absence, it was unable to draw a definitive conclusion," it said.
Russia and Syria have denied involvement in the bombing.
The board of inquiry, led by retired Indian general Abhijit Guha, was not allowed to visit the scene of the attack in Urem al-Kubra, but it did travel to Syria in early December.
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