Midway International Airport in Chicago lost power to its runway lights for about four hours on Thursday at just after 9:00 p.m. The airport was closed due to the power outage, and 85 flights were canceled or diverted during that time. Power came back at 1:20 a.m. the next day, and flights started again early on Friday.

Karen Pride, the spokeswoman for the Chicago Aviation Department, said that the outage was caused by a problem with the lighting equipment, according to Boston.com Yet, according to the Chicago Tribune, officials from the Chicago Department of Aviation said they are still trying to figure out why runway lights at the airport went out.

Pride told the Tribune via e-mail that an "extensive investigation" is underway. While officials acknowledged that an issue with the airfield's lighting vault cut power to the lights, but they couldn't explain anything else.

The high temperatures on Thursday might have been a factor, and backup equipment kicked in when the power went out, but Pride said that the alternate lighting system "was unable to sustain the runways' lights for long enough periods of time to ensure the safety of aircraft, passengers and employees at the airport."

Commonwealth Edison spokeswoman Martha Swaney elaborated, saying that the outage wasn’t caused by a ComEd power failure, but that crews from the company worked with Midway’s staff to get the airport's equipment working again.