Southwest Airlines said it fixed the technology problems that delayed hundreds of flights Sunday and that it expected a normal day of operations.
The Dallas company had been warning passengers flying Monday to arrive at the airport two hours early and print boarding passes beforehand. When asked early whether fliers should continue with those precautions, an airline spokesman said customers should expect a normal day Monday.
Southwest used backup systems around the country on Sunday to check-in travelers lacking printed or mobile boarding passes.
Airline representatives have not said what was causing the problem, but a spokesman has said there was no indication that hackers were behind it.
There were about 450 delays out of 3,600 flights scheduled Sunday.
Customers reported long lines at airports for check-in. Ida Perez said the Southwest check-in line at McCarren International Airport in Las Vegas, Nevada, wrapped around the building and down the street. Perez said she waited in line for over two hours before finally making her flight.
Meanwhile, other passengers took to Twitter to vent their frustrations that flights departed while they waited to check-in.
On Sunday, the airline apologized over the issue.
We apologize for this morning's technical issues. We are working to restore service to our Customers, and we appreciate your patience. — Southwest Airlines (@SouthwestAir) October 11, 2015