Nine passengers were hospitalized in Chicago, after falling ill before a flight.

It happened at O'Hare International Airport late Monday.

Emergency crews called five ambulances to the airport around 11:00 p.m. after a group of people about to board a flight reported feeling ill.

Seven of those passengers have been released from the hospital.

"You take a look at how many people were involved in the group, and you take the group out, isolate them and see if they developed any symptoms," said Doctor Drain Harnisch, Resurrection Medical Center.

The passengers were part of a tour group on the way back to Denmark.

The approximately 40 people that did not get sick were kept at a hotel as precaution.

The cause of the illness is currently under investigation.