The Obama Administration is stressing (correctly) that the only thing it has is evidence that some Syrians, somewhere, were exposed to some Sarin gas. That is a long way short of "evidence" that Assad launched an attack using Sarin gas. After all, the US military managed to expose its own soldiers to Sarin gas in 1991 when it blew up some Iraqi munitions without first checking what those munitions contained.....
47 killed in clashes between ISIS, Syrian rebels near Turkey border, monitor says (Reuters)
from the article: Pushing back the red line on Syria