I am really skeptical that Syrian forces did it.
First, Why will Assad do something that he will be condemned by many countries or worst be invaded by the West like in Iraw? Assad can finish all of the rebels w/o using chemical weapons and his troops liberated a lot of Syrian areas from rebels' control w/o the use of chemical weapons.
I know Assad is a brutal leader but not that idiot.
Second, The timing is really dubious, It happened when the rebels will be corned by Syrian troops.
Third, It is a clear fact that some rebels were armed by US and its Middle Eastern allies; who knows they secretly armed them with chemical weapons and will only use them if the rebels will nearly be exterminated/corned by Syrian troops.
I already know that the main reason for Syrian war is to depose Assad so that the US can help Qatar to build oil pipelines that will pass through Syria then to Europe so that European countries will never depend on Russian oil and this will enable Russia to be weaken more and isolated economically by the West.
This was also the main reason why Russia intervened for Assad.