A plane has been diverted to Tulsa International Airport after multiple passengers complained of sickness on Saturday. Delta Airlines flight 1817 was travelling to Denver from Atlanta when it diverted to Tulsa at about 3pm after the passengers apparently became ill at the same time. A Tulsa Fire Department official told ABC News some of the passengers who became ill had elevated levels of carbon monoxide in their bodies. However, Emergency Medical Services Authority spokeswoman Kelli Bruer said it's unknown what caused passengers to become ill. Bruer said responders with the ambulance service treated nine passengers suffering similar symptoms, including nausea. She said one passenger was taken to the hospital as "non-emergent" and another passenger for unrelated medical issues. The airline says 152 passengers were on board the plane when its crew decided to divert to Tulsa after "a few" passengers reported feeling ill. One passenger told Fox News that people were told to put their heads down, while others also vomited. An investigation into the incident continues.