GENEVA (Reuters) – Analysis of satellite imagery of a deadly attack on an aid convoy in Syria last month showed that it was an airstrike, a United Nations expert said on Wednesday.

Some 20 people were killed in the attack on the U.N. and Syrian Arab Red Crescent convoy at Urem Al-Kubra near the northern city of Aleppo which destroyed 18 of 31 trucks, a warehouse and clinic.

The United States blamed two Russian warplanes which it said were in the skies above the area at the time of the incident. Moscow denies the charge and says the convoy caught fire.

“With our analysis we determined it was an airstrike and I think multiple other sources have said that as well,” Lars Bromley, research adviser at UNOSAT, told a news briefing.

“For airstrikes, what you are usually looking out for is the size of the crater that is visible and the type of crater,” he said. A giant crater on the ground was caused “almost certainly (by) air dropped munitions” as opposed to artillery or mortars, he said.

The United Nations thus far had referred only to an “attack,” which led to a brief suspension of its land convoys in Syria, while the International Federation of Red Cross and Red Crescent Societies initially referred to “airstrikes” in a statement.

U.N. Secretary-General Ban Ki-moon announced on Friday that he would establish an internal U.N. board of inquiry to investigate the attack and urged all parties to fully cooperate.

UNOSAT, which reviews only commercially available satellite images, has not been asked to share its analysis with the U.N. inquiry, but it is prepared to do so, its manager Einar Bjorgo said. “Our images are from time to time used in order to brief Security Council members,” he told the briefing.