Claims that Assad is using chemical weapons are like a barometer: when the Syrian army is doing well, they surface, notably in 2013, 2015 and now, just as the Syria government looks close to some kind of 'victory'. Both times in the past the intelligence came from Mossad and the claims fizzled out, though the propaganda that it was 'likely' by the Syrian Army stuck in western perception. The current chemical 'attack', instantly hailed by Israel, occurred just as peace talks were beginning in Geneva. The source of the claim is, again, most likely Israel, though that's not part of the media fireworks. Tillerson might have checked with the Russians, as Russian military were stationed at the airport.

That is the background to the bombing of the air base April 6,  in retaliation for a suspected chemical weapons attack on civilians in rebel-held Idlib province two days before. National security adviser General Herbert McMaster solemnly declared,  "We could trace this murderous attack back to that facility.” Secretary of State Rex Tillerson accused Russia of being either complicit or incompetent in failing to keep its 2013 promise of completely destroying Syria’s chemical weapons supply.