Police and first responders are taking part this weekend in a training exercise at Chicago's O'Hare International Airport involving a mock hostage taking.
Sunday morning's drill is taking place in one of the airport's parking lots and an adjoining station of the O'Hare Airport Transit System.
The rail system ferries passengers between O'Hare's terminals and parking lots.
The mock hostage incident will take place on the transit system.
The Chicago Department of Aviation says the exercise will require the closure of the "Kiss 'n Fly" lot from midnight to noon.
Travelers and employees will still be able to use the facilities but access may be limited in some areas.