"credible sources" - ???  same "doctors and aid agencies" who are integrated with the Salafist jihadist rebels. "Turkish authorities" also represent interests who most definitely identify with a particular side. There has been a great deal of disinformation and sketchy attribution throughout this conflict. The Ghouta chemical attack from 2013 was similarly portrayed as an obvious government-led attack, but most of those claims have been severely challenged in the time since. In fact, the only documented and verified responsibility for a chemical attack in Syria has been identified with the rebel groups - confirmed by information published by the UN working group. 

Fool me once, fool me twice.... WMD, Libya attacking civilians, Assad gassing his people... wag the dog. So far, in this case, the absence of evidence as to what caused the event is glaring. People making assertions is not evidence.