Southwest Airlines said hundreds of flights have been delayed by technical problems and warned passengers flying Monday to print boarding passes before going to the airport.
The Dallas company said it was using backup systems around the country to check in travelers lacking printed or mobile boarding passes, but technology problems that began Sunday morning were continuing. On Sunday, about 450 of the 3,600 scheduled flights were delayed.
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Representatives for Southwest didn't say what caused the problem, or how much longer it was expected to last. Spokesman Brad Hawkins said there was "absolutely no indication now" that the problems were the result of hacking.
The airline has asked travelers to arrive at least two hours before their scheduled departures.
At Los Angeles International Airport earlier in the day, several dozen people crowded the Southwest terminal waiting to be issued handwritten tickets.
Emily Mitnick, who was flying to Detroit from Denver International Airport, said she missed her 10 a.m. flight, even though she parked her car around 8 a.m. She estimated that 1,000 people were in line at the check-in for a boarding pass. When she went downstairs to the curbside check-in, she said there were about a couple hundred people lined up there as well.
By the time she got to security, it was about 10:15 a.m.
"The clock was ticking and the flight took off," said Ms. Mitnick, who was trying to get to Detroit through a different flight to Chicago.
Last month, American Airlines experienced computer problems that prevented passengers from checking in and briefly halted flights on select routes. Airline officials said at the time that they fixed the problem after less than two hours, and that there was no indication that its system had been hacked.
Southwest is the top hauler of domestic passengers and the fourth-largest U.S. airline by traffic.