Who attacked the aid convoy in Syria on September 19, 2016?

On September 19, 2016 at around 7 PM local time, a UN aid convoy carrying vital supplies for tens of thousands of Syrian civilians in a besieged area south-west of Aleppo was attacked as it was being unloaded at a Syrian Arab Red Crescent (SARC) warehouse in Urum al-Kubra. 18 civilians were killed, 18 of the 31 trucks were completely destroyed, and neighboring buildings, including the warehouse and a clinic, were damaged in the three-hour attack. Following the attack, Russia and the United States blamed one another. Russia offered a number of claims, some contradictory. The UN and Human Rights Watch both called for an investigation and raised the possibility that this was a deliberate attack on civilians, and thus a war crime. The attacks highlighted the collapse of a fragile week-long cease-fire that had been supported by Russia and the US.