It may have been the rebels, it may have been one of Assad's warlord-like generals, it may have been Assad himself that ordered it.
Thing is, we have very recently caught the intelligence community and the NSA in particular OUTRIGHT LYING under oath to Congress. Even if they have the evidence they claim to have making out a circumstantial case that it was the pro-regime forces that used it, how do we know the so-called evidence is genuine as opposed to being the product of one of their own black ops in Syria, or even just outright fabricated?