TWO days after the alleged chemical attack on the Damascus suburbs of Eastern Ghouta, chemical-weapons experts are dissecting amateur footage to determine exactly what might have caused the deaths of so many hundreds of Syrians. All agree this time, unlike in pastalleged attacks, that the number of victims and the lack of marks from physical wounds on their bodies point to some form ofchemicalpoisoning. But they are puzzled that the symptoms—insofar as they are visible from the videos—do not exactly correspond to any particular known substance, including the large quantities of mustard gas, sarin and VX which President Bashar Assad is thought to have at his disposal. “It is beyond doubt that something has made a lot of people ill and killed them,” says Dan Kaszeta, a chemical and biological expert who now runs Strongpoint Security, a defence consultancy. “But there is no obvious agent.” The gamut of possible chemical weapons includes blister agents such as mustard gas, blood agents known as cyanogens, choking agents like chlorine or phosgene, and nerve agents such as sarin and VX. The first three can probably be ruled out in this case, reckons Michael Elleman, a Bahrain-based senior fellow at the International Institute for Strategic Studies, because the footage shows no evidence of blistering, or the blue tinge of skin brought on by blood agents, and the speed of death of many of the victims was faster than would be the case had choking agents been used. The latter are also very smelly, and no witness reported a terrible stench. Many observers have concluded that the likely culprit was a nerve gas. Hamish de Bretton-Gordon, a former commander of British chemical and biological counter-terrorism forces, believes that "in the main" that is what the symptoms suggest. But Mr Elleman says that given that these agents cause loss of muscle control, he would have expected to see more urination and defecation by victims. Jean-Paul Sanders, a French chemicals expert, has written that he would have also anticipated more convulsions in the case of a nerve agent.

So what was used? Some contend that a toxic industrial chemical may have been deployed. But Mr Elleman has another theory: “I think there is a reasonable possibility that a nerve agent past its shelf life was used,” he says. “That would explain why the symptoms do not match exactly.” As an example, he cites Iraqi stores of VX gas—a very toxic substance but which the Iraqis struggled to make in a pure form, meaning it only lasted six months, after which it became less poisonous. Mr Elleman says it is possible that a degraded nerve agent could cause its victims to exhibit fewer of the usual symptoms and still die just as quickly.

The August 21st attack appears to have been rendered more deadly by Syrians’ attempts to flee the bombardment. As a doctor in Jobar, an area of Eastern Ghouta, explains, many hid in basements. But because chemical agents are heavier than air they sink to lower-lying areas, where there is in addition little ventilation.

If UN weapons inspectors, already in Damascus to investigate three alleged past instances of chemical attacks, are allowed access to the site of the latest incident, the mystery could quickly be solved. But that seems unlikely for now. In the meantime, the laborious effort to piece together a picture from the outside continues.