The Independent International Commission of Inquiry on the Syrian Arab Republic, has issued its 7th report which deals with many atrocities committed by all parties to the conflict, but also provided the following interesting quote (page 19):
The evidence available concerning the nature, quality and quantity of the agents used on 21 August indicated that the perpetrators likely had access to the chemical weapons stockpile of the Syrian military, as well as the expertise and equipment necessary to manipulate safely large amount of chemical agents.The report also states that the chemical agents used in Ghouta were similar to the Khan Al-Assal attack, and concludes that "In no incident was the commission’s evidentiary threshold met with respect to the perpetrator".
This seems to indicate the commission had information that could prove valuable for our research here. Unfortunately, UN bodies have proven in the past to be a problematic source of information when dealing with the Ghouta attack, so some care should be practiced. Indeed, the first thing that stands out is that the commission does not provide any evidence to support its conclusion, or even provide an indication of what that evidence could be. This obviously renders this information unusable for our investigation.
Nevertheless, we can try to deduce what information the commission has by analyzing their statements.
A first clue is given in the first question in the press conference that followed the report, where the commission's chairman explains that since they did not visit Syria, their conclusion is based principally on the existing findings of Sellstrom's team, as well as "interviews with experts and functionaries".
A second clue is the comparison to the Khan Al-Assal attack, which is said to have "the same unique hallmarks as those used in Al-Ghouta". Since Sellstrom's team did not visit Khan Al-Assal and had no field samples, the only source for such a comparison would be Russia's 100-page report of the attack submitted to the UN, which included certified lab results of field samples. The full results were not published, but were reported to provide evidence that RDX was used as the bursting charge.
These clues bring us back to the well-known Hexamine issue - which serves as the current "smoking gun", ever since the "trajectory intersection" theory was refuted. Since RDX is based on Hexamine, it seems likely that the Russian labs reported Hexamine in their samples, which brought the commission to connect them to the Hexamine in the Ghouta attack (probably correctly), and both attacks to the Syrian stockpiles (probably incorrectly).
So since the commission refuses to provide any evidence, states that their conclusion is based on Sellstrom's data, and found similarities to Khan Al-Assal, there is little reason to believe they have more information besides the well-known Hexamine finding.
The other evidence mentioned is the amount of agent used, which is indeed one of the main challenges to the opposition-attack theory. However, detailed analysis indicates that while not an easy feat, producing such amounts is within the opposition's capabilities. The recent UN evidence showing the opposition deploying tens of kg of sarin, further strengthened this position.
As a side note, for those who are not familiar with the UN's multiple manipulations during the Ghouta investigation and choose to take this report at face value as indicative of a government attack, it should be pointed out that this would indicate that the only two cases where the government chose to use sarin at large scale are (a) an attack against Syrian soldiers in a government-controlled area, and (b) a massive attack on a purely civilian opposition neighborhood carried out upon arrival of a UN team invited to investigate the first attack. - An obviously perplexing choice of targets.
Finally, it is important once again to remember that the Ghouta attack is no longer a mystery where each little clue can change the picture (like it was in the early days). We now have very strong evidence implicating the opposition in the attack, and so far no one was able to propose a regime attack theory that is consistent with the evidence (see the end of this post to understand why this is so difficult). For our understanding to change, very strong contradicting evidence should be brought forward, or alternatively a plausible theory for a regime attack should be formulated. Vague statements or general-purpose chemicals claimed to be smoking guns, sadly, do not qualify.
Conclusion: The commission's quote seems to be based on the Hexamine findings and the amount of agent used, both of which have already been known and analyzed here before, and found to be of little value.