See also my April 24 post, A neuropharmacologist debunks use of sarin in the Khan Shaykhun incident. The evidence presented is staggering when one considers the number of governments and news organizations that have ignored the true symptoms of sarin poisoning. The evidence makes it impossible for me to give Gen. Mattis the benefit of the doubt, as I did in this earlier post.
Questions for Secretary Mattis. Assume it was indeed sarin gas that was released in Khan Shaykhun, as the U.S. intelligence briefing that you referenced yesterday claims. What kind of sarin? Homemade sarin? Military-grade sarin? Now how would you be able to tell the difference between the two?
The answer is that you don't know how because if you knew, you would not have attested that the Syrian government was responsible for a sarin attack in Khan Shaykhun.
Unless you were lying; that is, you were aware of the kind of investigation it would take to demonstrate the claim and so knew that between the date of the chemical incident and the ordering of a US bombing of a Syrian air base, there wasn't enough time for an investigation.
If you were lying, you were taking advantage of the public's ignorance of how investigations of chemical weapons proceed. And, I suppose, hoping that a compliant news media wouldn't highlight the explanations from any chemical weapons experts who debunked the briefing you referenced.
As to the other explanation -- ignorance -- just how much intelligence does it take for a person to realize he's out of his depth? So even if it didn't occur to you that analyzing a chemical weapon might be more complicated than analyzing, say, the chemical composition of cheese tostitos, you do know how to use a phone to call for advice, do you not?
For those who want to gain an idea of what it takes to analyze a chemical weapon and determine its provenance, study David Habakkuk's Sentence First, Verdict Afterwards; April 9,2017, Sic Semper Tyrannis. I warn that the paper is not easy reading but at the least you'll learn from it that analyzing a chemical weapon is a lengthy, complicated process.