Some of the passengers who became ill on a Delta flight from Atlanta to Denver Saturday afternoon -- prompting its diversion to Tulsa, Oklahoma -- had elevated levels of carbon monoxide in their bodies, a Tulsa Fire Department official said Saturday evening.

The cause of the increased carbon monoxide levels was not yet determined. An investigation continues.

Once passengers went outside and got fresh air, their levels returned to normal, said fire department spokesman Stan May

"The flight crew of Delta flight 1817 from Atlanta to Denver elected to divert to Tulsa when a few customers reported feeling ill," the Atlanta-based airline said in a statement Saturday afternoon. "The safety and security of our customers is our top priority."

Initially, officials at Emergency Medical Services Authority in Tulsa said 100 people had been assessed, and nine reported having the same symptoms -- but the cause was unknown.

Then later in the day, May released the information about the elevated carbon monoxide levels, adding that the number of people complaining of symptoms had risen 12. One person was taken to a hospital, but he said it was not related to the carbon monoxide.

Officials were re-screening passengers at about 8:30 p.m. local time to get them on another Denver-bound plane.