On Friday the OPCW's fact-finding mission issued its final report on alleged chemical attacks in the rebel-held Syrian town of Douma last April.
Dozens of people were reportedly killed by a release of toxic chemicals and the US, Britain and France responded shortly afterwards with airstrikes targeting the Assad regime's chemical facilities.
Although the regime was widely believed to have used chemical weapons in Douma, the regime – together with its chief ally, Russia, and various western conspiracy theorists – claimed rebels had faked the attacks in order to blame the regime.
In an interim report last July, the OPCW said laboratory tests had found no evidence that sarin or a similar nerve agent had been used but found possible evidence of chlorine use.
There is substantial evidence that chlorine has been used as a weapon elsewhere in Syria during the conflict. According to a previous OPCW report it was used “systematically and repeatedly” as a weapon on villages in northern Syria.
The 106-page report issued on Friday points to two main conclusions regarding Douma – that the chemical involved was chlorine and that the gas came from cylinders which had been dropped from the air.
The significance of this – although the report doesn't spell it out – is that the cylinders must have been dropped by pro-Assad forces, since the rebel fighters had no aircraft.
Based on all the available evidence, including witness testimony, environmental and biomedical samples and toxicological and ballistic analyses, the report says there are "reasonable grounds" for believing a toxic chemical was used as a weapon, and it adds: "This toxic chemical contained reactive chlorine. The toxic chemical was likely molecular chlorine."
On the question of delivery methods, the report looks in detail at two yellow cylinders found in Douma. It describes these as "industrial cylinders dedicated for pressurised gas" about 140cm long and 40cm in diameter.
One cylinder was found lying on a top-floor patio where its impact had apparently punched a hole through into a room below. The other cylinder had apparently crashed through a roof into a bedroom, ending up on the bed.
Images of the two cylinders were circulated on the internet shortly after the attack. Assad's supporters claimed they had been planted by rebels rather than falling from the sky and suggested the roof damage seen in the images must have had a different cause.
However, the OPCW's report dismisses that idea. It says:
"The team analysed the available material and consulted independent experts in mechanical engineering, ballistics and metallurgy who utilised specialised computer modelling techniques to provide qualified and competent assessments of the trajectory and damage to the cylinders ...
"The analyses indicated that the structural damage to the rebar-reinforced concrete terrace at Location 2 [the top-floor patio] was caused by an impacting object with a geometrically symmetric shape and sufficient kinetic energy to cause the observed damage. The analyses indicate that the damage observed on the cylinder found on the roof-top terrace, the aperture, the balcony, the surrounding rooms, the rooms underneath and the structure above, is consistent with the creation of the aperture observed in the terrace by the cylinder found in that location.
"At Location 4 [the bedroom], the results of the studies indicated that the shape of the aperture produced in the modulation matched the shape and damage observed by the team. The studies further indicated that, after passing through the ceiling and impacting the floor at lower speed, the cylinder continued an altered trajectory, until reaching the position in which it was found."
Widely-circulated images show that both cylinders had been fitted with a metal harness to provide them with wheels, lugs for lifting, and tail fins. The addition of tail fins is especially important because, according to a previous OPCW report, this shows the cylinders were intended to be dropped from a height, with the fins stabilising their descent. In other words, the fins confirm they had been adapted for use as a weapon.
The same OPCW report also said that if weapons dropped from the sky were "not designed to cause mechanical injury through explosive force" it was reasonable to conclude that they were chemical weapons – and this is clearly the case with the Douma cylinders.
The fact-finding mission's mandate is to determine whether chemical weapons or toxic chemicals as weapons have been used in Syria, and this does not include identifying who is responsible for alleged attacks. However, as a result of a decision last June the OPCW itself is authorised to identify those responsible and is likely to address that at a later stage.