if it was mustard gas, it can take up to 12 hours for symptoms to appear.
http://www.nationalterroralert.com/mustard/
luckily, it's not usually fatal.
Interesting. I had always assumed that mustard gas had immediate symptoms.
Okay, I lower my assessment to the opinion that there's a 99.9% chance the attack never happened, which can be raised to 100% within a few minutes.