The U.S.-based United Airlines said on Thursday that its check-in system is back up at O'Hare International Airport in Chicago after hours of chaos-causing failure, and that the company is still investigating what caused the computer outage at its largest hub.

A system problem involving all of United's computers at O'Hare caused flight delays and cancellations on Thursday morning. Passengers were asked to continue checking flight information and checking on their flights through the website, united.com.

Chicago passengers looking to change their travel plans can do so without additional fees at the website, a statement from the airline said.

At the peak of the problem, at least 1,000 passengers waited inslow-moving lines at the O'Hare terminals. Planes lined up on the tarmac as new flights arrived yet departing flights were unable to leave.

O'Hare International Airport is a major hub for United Airlines, and the problems have been causing delays at other airports in the United States as well.

United Airlines had to ground all flights of O'Hare International Airport this morning because of what it called an "airport check-in system issue." And incoming flights were delayed.

However, at about 8:45 a.m., automated self-service check-in kiosks began issuing boarding passes again at the airport. About 75 percent of check-in computers were back in service about 10:30 a.m. and the rest were up by 11 a.m..

Shortly after 11 a.m., United Airline's schedule at O'Hare International Airport was returning to normal, with very few scheduled departures delayed substantially or canceled.

Source: Xinhua