Business as usual: no proof (here about chemicals), blame your enemy for whatever happens in the world and arrange a strike.
"Déjà vu" in Yugoslavia, Iraq, Lybia, etc.
IS and so-called rebels often used chemicals both in Syria and Iraq (either self-made or stolen long ago from the regime before they were eleminated) but these facts were ignored and not broadcast in western "mainstream media". IMHO, it does not sound absurd that some of their stocks was blown up during the bombardment and poisoned an area around. However, I do not line out the possibility of the Assad's men involved (don't like that guy). In fact, it looks not so difficult to investigate what actually blew up that contained sarin (or whatever was inside), but who bothers? The side to be blamed was assigned in advance.