Actually, Syria’s long been known to possess chemical weapons and it’s also known to have accepted Iraq’s chemical weapons precursors and equipment when the Russians helped transport them to Syria in advance of the US invasion of Iraq [see Jack Shaw’s testimony]. The most likely explanation is the weapons are Syrian or Iraqi [and therefore Russian in origin] and they were used by the Syrian government which bought and paid for them in the first place, and the Russians would of course try to promote the idea that they are not responsible even though they certainly liked the money they took for selling the stuff.
Actually at Russia's suggestion, Syria voluntarily got rid of its chemical weapon stockpiles a few years ago under UN supervision. Please adjust your story line to include those facts.