Last updated 9:55 a.m. EDT
A computer problem grounded all United Airlines flights at O'Hare International Airport Thursday, causing long delays and lines for travelers headed out for the Fourth of July holiday weekend.
The outage involved all of United's computers at O'Hare, forcing many delays and some cancellations, said airline spokeswoman Robin Urbanski.
But United told CBS News that the problem is limited to O'Hare.
Because the outage involved United's ticketing system, the airline urged customers to monitor their flight status and check-in online before they got to O'Hare, Urbanski said.
United employees at the airport used a manual process to check customers in, she said.
Karen Pride, spokeswoman for the Chicago Department of Aviation, said early Thursday that she didn't know how many flights were affected or how long the delays were.
O'Hare is a United hub, meaning the problems could cause delays at other U.S. airports.
The O'Hare terminal is packed and passengers described the scene as a nightmare, CBS station WBBM reports.
Passengers reported long lines and planes lined up on the airport's tarmac as new flights arrived and departing flights were unable to leave.
Lois Norder told the Chicago Tribune that the pilot on her flight to Dallas announced that they couldn't take off because the computer problems were keeping crews from refueling the plane.
"The employees are really stumped," Norder said. "They're trying to figure out what to do."
Click here for the full WBBM report.