Thursday afternoon, investigators were still trying to determine what caused the 59-year-old contractor to fall to his death.
Coworkers say the accident happened just before 10 o'clock Thursday morning when a contractor fell to his death as he worked on the outside of Terminal 5 at O'Hare.
The worker was killed after falling 35 feet off a roof of the airport building to a lower level.
Thursday afternoon, authorities say it is still unclear what caused the fall.
A spokeswoman for Chicago's Department of Aviation says the man was not a city employee.
Other airport workers say the man was an electrician who was a part of a team of workers - working for a private company responsible for performing maintenance on the electrical systems at Terminal 5 when the accident happened.
The victim, a 59-year-old father of two, was critically injured by the fall and was taken to Lutheran General Hospital in Park Ridge where he was later pronounced dead.
Co-workers said the man worked for the private contracting company for about 12 years.
Investigators said they are also looking into whether safety protocols were followed.