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By The Associated Press and Tim Stelloh

Southwest Airlines says it's fixed the technology problems that delayed hundreds of flights Sunday and that it expects a normal day of operations on Monday. But long lines remained at some airports early in the day.

Before issuing a statement, the Dallas-based company had been warning passengers flying Monday to arrive at the airport two hours early and print boarding passes beforehand. When asked by the Associated Press early in the day 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.

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Airline representatives have not said what caused the "technology issues," which affected the airline's mobile app, website and reservation centers. but an airline spokesman said there was no indication that hackers were behind it. The problem forced airline workers to manually issue tickets.

There were about 450 delays out of 3,600 flights scheduled Sunday, causing long delays and massive lines at airports from Los Angeles to Washington, D.C.

In a statement posted on the airline's website, Southwest pleaded with customers to arrive two hours before departure.

In a tweet to one customer asking about a flight out of Oakland, Southwest described the issue as "systemwide."

@sir_bester Hi Scott, it's systemwide and we are doing our best to resolve these issues quickly. ^LT — Southwest Airlines (@SouthwestAir) October 11, 2015

Photos from Los Angeles and Phoenix showed airports jammed with people; one customer at Sky Harbor International in Phoenix described it as "Disneyland on a weekend." At LAX, airport operators #LAXTRAFFIC %20LAX%20OPERATIONS%20will%20be%20providing%20water%20and%20canopies%20for%20Southwest%20passengers%20waiting%20outside%20terminal%201.%20pic.twitter.com/N2kICVAdS2