The United States says it is reserving judgement on whether chemical weapons were used in the attack on a suburb of Damascus on Wednesday, which led to widespread casualties.
But as a clearer picture begins to emerge of the alleged attack and its consequences, some experts say they are becoming more convinced that a nerve agent may have been used.
Dozens of amateur video reports are now available online about the alleged chemical attack. Although the material is unverified, it helps provide a fuller picture of what may have happened on the outskirts of Damascus in the small hours of Wednesday morning.
In the first place the timing of the attack is becoming clearer: it was in the dead of night.
'Chemical attack': What we know
- 01:15: 21 August (10:15 GMT 20 Aug): Facebook pages of Syrian opposition report heavy fighting in rebel-held districts of Ghouta, the agricultural belt in eastern Damascus
- 02:45: Opposition posts Facebook report of "chemical shelling" in Ein Tarma area of Ghouta
- 02:47: Second opposition report says chemical weapons used in Zamalka area of Ghouta
- Unverified video footage shows people being treated on pavements in the dark and in a makeshift hospital
- Reports say chemical weapons were used in Ghouta towns of Irbin, Jobar, Zamalka and Ein Tarma as well as in Muadhamiya to the west, but this is not confirmed
- Syrian government acknowledges military offensive in the Ghouta area but denies chemical weapons use
Some of those who did not survive can be seen laid out in their nightclothes in the basement shelters where they were taken. Some survivors describe being woken up by the blasts in the middle of the night while they were in bed.
"We were asleep when we were hit," says one small boy. "My mother put wet clothes on our eyes, it was burning. My father was screaming 'Get out... get out'. We saw a dead body outside as we left the house. My mother fainted and my father started crying. He put me in a car and the car left. I don't know where he is."
"I don't know where any of them are," he adds, before dissolving into tears.
Timing and location
One video report shows the headlights in the pitch dark of what appear to be ambulances with sirens screeching, apparently rushing from areas that had been under attack.
Another report shows victims being laid out on a pavement to be methodically washed down in an apparent attempt to decontaminate them. A low light, as though from car headlights, shakily illuminates the night time scene.
And inside one makeshift hospital, where victims are being frantically treated, the call to morning prayer can be heard - a call which usually comes some 45 minutes before sunrise.
Even more precise timing is suggested by posts on Facebook.
On the three main Facebook pages of Syrian opposition groups, the first mention of chemical weapons came at 02:45 local time.
A report from the Ein Tarma Co-ordination Committee announced that "a number of residents died in suffocation cases due to chemical shelling of the al-Zayniya area [in Ein Tarma]".
Two minutes later at 02:47, the Sham News Network posted an "urgent" caption, and claimed that government forces had shelled Zamalka using chemical weapons.
The third post, from the Local Co-ordination Committees (LCC), an opposition activist network, came minutes after at 02:55 with a similar report.
Significantly, an hour and a half earlier at approximately 01:15, the three groups had posted reports of fierce clashes in the same areas of the eastern Ghouta, the agricultural belt around Damascus, between Free Syrian Amy rebels and government forces, as well as shelling by government forces, and a claim that the Free Syrian Army had shot down a helicopter in the area.
The alleged chemical attacks on civilians that have so shocked the world did not, it seems, come out of the blue.
As for the location of the attacks, time and again rebel strongholds in the eastern Ghouta are mentioned, especially the towns of Irbin, Jobar, Zamalka and Ein Tarma. And to the west of Damascus, the town of Muadhamiya is named.
And from the overwhelming and distressing litany of footage of victims an overview of the symptoms can be gleaned.
Most of those being treated are men of all ages and very small children.
Few women have been filmed, perhaps out of respect for their privacy, possibly because they were less likely to have been sleeping on the roof in the open air, and therefore less likely to be exposed to toxic fumes.
Among those laid out on floors, or being given assistance on makeshift beds and stretchers, there are none who show any outward sign of blood or lacerations. But there are many who have extreme difficulty breathing and are being helped with oxygen masks.
One man twisting and shivering on the floor seems to be having convulsions. Several are in such distress, they seem to be foaming at the mouth or nose. One man whose stark, glazed eyes stand out from his ashen face looks almost frozen, his pupils apparently contracted - a telling indication of possible nerve gas.
Weapons and delivery systems
Even though the US government says it is still not able to say conclusively that chemical weapons were used, Stephen Johnson, a former British Army Chemical weapons expert now attached to Cranfield University's forensics department, said the mounting visual evidence seemed to point to the use of chemical weapons.
"The scale of this, the number of people who seem to be affected from the videos, the consistency of the symptoms, that's a staggering enterprise to fake - not only staggering to fake but also very easily found out once an investigation takes place."
In many quarters, however, the debate seems to be moving away from whether a chemical attack took place to which side might have been responsible. And here Stephen Johnson says an examination of the weapons and delivery systems will be crucial.
"It doesn't seem like the Russians are contesting that a nerve agent has been used, so it is more important that we determine where this has come from and who fired it.
"It's really critical to get onto the ground to see the impact sites as soon as possible. You need to try and find the remnants of theses rockets and see if they are consistent with the pictures we have seen. Outside of 14 days it's going to be really difficult to examine these scenes."