I quite remember the US fabricating the same thing under President Obama saying Assas used chemical weapons, they whole thing was later revealed to have been fabricated and that the rebels used the chemical weapons and the US knew about it the whole time, just that they wanted an excuse to oust Assad.
In the Idlib province (where Khan Sheikhoun is located), infighting between the various rebel groups have been ongoing for the past several months. Many of these groups were in the possession of chemical weapons. It is difficult to ascertain who used which weapon now, but the Americans used the incident to destroy the Syrian aircraft.