Till now, there is no solid proof to blame the Assad regime for the chemical attack. If they have any sort of evidence, then let them release it to the public domain. IMO, the attack was committed by either the FSA, or the ISIS.
When it stopped Americans? If they put Assad guilty, they will do everything to remove him from power. Chemical weapons is only a pretext which, if necessary, can change.
Who then do you think all this was done? Who had chemical charges? Only Assad could have made such a decision.