Do you read the articles you cite? Matthis references past poison gas attacks from the Syrian Assad Shi'a Muslim government. Your article is Matthis saying in 2018 that recent events have no proof (yet), but Matthis in the article you link to warns Assad not to repeat past use of poison gas. "Mattis says it is clear that Assad’s government has weaponized and used chlorine gas in the Syrian civil war. ..Mattis alluded to the April attack, saying, “"So they’d be ill-advised to go back to violating” the international prohibition on the use of chemical weapons."" Your AP article from 2018 is entirely consistent with the Obama White House officially saying in 2013 that it has high confidence that U.S. Intelligence concludes that the Syria Assad Shi’a Muslim government had used chemical weapons at that time years earlier.