I'm not sure it was Sarin. If it was a different gas, such as chlorine or Phosgene, availability is less of an issue. There have also been reports that this was fallout from an earlier strike against a weapons depot causing the accidental release of gasses from stored chemicals - perhaps weaponized, perhaps not.

If there needs to be a trial, isn't that the alleged purpose of the UN? It's not 'MURKA's job to be judge, jury, and executioner regardless of precedent.