The cause of the deaths of hundreds of Syrian civilians in the early hours of Wednesday in the eastern suburbs of Damascus is still uncertain.

The Syrian rebels claim that they were killed in a chemical weapons attack carried out by the Syrian army, but some chemical experts are skeptical.

As of Wednesday, very little was known about the attack on the Ghouta neighbourhood, not even the number of casualties. According to some sources, the number could be as high as 1,300.

What is clear is that a large number of civilians, including many children, were killed without signs of external trauma. This indicates suffocation or another respiratory cause, and the rebels claimed that the civilians had been killed by land-based missiles with warheads containing Sarin nerve gas.

Israeli defence sources also said that they detected signs of a chemical attack but no official was prepared to say so on the record. Defence Minister Moshe Yaalon said in a briefing on Wednesday that “Syria has used chemical weapons and not for the first time” but did not specify which event he was referring to or which weapons had been used.

Western experts voiced their reservations based mainly on the lack of contamination evident in the footage.

Dan Kaszeta, a former officer with the US Army’s Chemical Corps, said that “none of the people treating the casualties or photographing them are wearing any sort of chemical-warfare protective gear. And despite that, none of them seem to be harmed.”

This could rule out most types of military-grade chemical weapons which, if used in large quantities, leave traces on clothes and bodies which would harm anyone coming in contact with them in the hours after an attack. An alternative explanation was that those who died could have suffocated as a result of a vacuum caused by fireballs created by large conventional bombs.