Nor can you say it with any certainty that it was.  All the evidence shows that they blew up a factory making gas.  They have nothing to gain from gassing their own people. 

And think of it this way, If I'm going to use gas, I'm going to use it ALL THE TIME! Not twice in a 6 year war to kill off  some civilians. This doesn't make sense. This isn't strategically helping anything, and it can get the world on your back if you use gas, so why risk it? Bombs are effective, please understand this is all propaganda to get Assad out because he doesn't back the West.