There's no solid proof yet as to who used the gas but It's not very hard to guess it out imho. It might be the insurgents who used the gas on the supporters of their own, but how/where could they make the gas to use, in the first place? it's like when you return home in the morning from a week-long trip on business, a few days ahead of schedule, and happen to find your wife lying in bed naked with another dude who's also clothless, you don't need any evidence to know what had happened the night before, or the week before.