Looks like a dozen or so Delta passengers got sick from CO ingestion. How would something like that happen? Like, what would have to go wrong with the aircraft in order for toxic levels of CO to leak into the cabin?
I once heard about some kind of generator whose exhaust plumbing allowed cabin inhabitants to get sick. It sounded like something similar in principle to a portable gasoline generator being used. For some reason, maybe to let the engine operate closer to standard sea level atmospheric pressure, this thing was inside the cabin. Its exhaust was plumbed to the aircraft exterior but something malfunctioned with that.
I have no inside info, but sounds like something happened to the APU:
Additional coverage online says no elevated CO level was found on the plane, and no source to explain such a finding was uncovered.
One source says the Tulsa F.D. used pulse oximetry to measure the supposedly elevated CO levels, which can give false positive readings.
http://airport.blog.ajc.com/2016/07/11/delta-flight-with-carbon-monoxide-scare-still-a-mystery/
Without confirmatory blood testing (if it was done, which sounds doubtful), the mystery includes whether or not the passengers actually had elevated CO levels, and why passengers in the front of the plane reportedly did not get sick.
I would bet the Carbon Monoxide entered the plane while it was still on the ground. And would have come from a running ground vehicle, generator, or connection to airport air conditioning.
Did the passengers get sick right after the plane took off?