1. The Syrian government has been winning the "civil war" so there is hardly any need for dramatic and desperate measures.
2. The so-called "rebels" are the ones who've threatened chemical attacks and have been caught engaging in them. Syria, which has been at war several times in the last five decades, has no record of using them.
3. It would be utterly irrational not to mention personally suicidal for Bashar al-Assad, the leader of Syria, to use such weapons. He's shrewd enough to know that by doing so he would invite UN sanctions and a NATO/US invasion.
So if the US/State Department line is not credible then what is?
We now know that the Spanish did not sink the USS Maine in Havana harbor, but that incident was enough to start the Spanish American War - and give the US a reason to take Cuba and the Philippines from the weak hand of Spain.
We also now know there was ample warning that the Japanese were preparing an attack on Peal Harbor and that this information was not passed on to the commander there.
The Gulf of Tonkin "attack" on US naval vessels off the coast of Vietnam has been demonstrated to have been a fraud and the recent War in Iraq was based on the existence of "weapons of mass destruction" that turned out not to exist.
The staging of incidents - including incidents that result in the death of innocent people - in order to justify military actions being carried out for entirely different motives is not a new thing and, as outrageous as it may seem to normal, decent minded people, it's not uncommon.
Is Syria another case of this?
We don't have the answer, but if we were betting, we'd bet on James Corbett's analysis on this one.