United Airlines said a balky check-in system that played havoc with its schedule at O'Hare International Airport was almost back to normal Thursday.

A few delays remained and some flights were canceled but generally the computer glitch that held up check-ins for several hours had been resolved, the Chicago Tribune reported.

About 2,000 customers were in a line that stretched from the ticket counter outside of the terminal at the height of the problem early Thursday.

United said it was trying to determine the cause of the computer outage at its largest hub on one of the busiest travel days of the year, the eve of the long Fourth of July holiday.