McCain made more than one secret trip to Syria.
It appears that in May of 2013 he met with rebels east of Azaz. Three months later, on August 21st, was the Gouda chemical weapons attack.
He was back in February 2017, meeting with Erdogan in Turkey and with rebels in Northern Syria (possibly at the border crossing). Two months later, in April, a chemical attack in Idlib prompted Trump to crater that runway.
Where McCain goes, chemical attacks seem to follow.
sauce: http:// theduran.com/every-time-john-mccain-makes-secret-trips-to-syria-chemical-weapon-attacks-follow/