American Airlines flights to Dallas, Chicago and Miami were briefly stopped on Thursday by a computer problem that prevented passengers from checking in.
Airline officials said they fixed their computer systems after less than two hours but were still trying to determine the exact cause of the interruption.
American spokesman Casey Norton said there was no indication that the airline's computers had been hacked.
The midday failure affected flights on American and its regional affiliate, American Eagle. The Federal Aviation Administration said that American Airlines planes destined for Dallas-Fort Worth International Airport, O'Hare Airport in Chicago and Miami International Airport were held on the ground at other airports.
Norton said six American Eagle flights in Chicago were canceled and about 300 American and Eagle flights were delayed by an average of slightly more than one hour. Tracking service FlightAware.com reported nearly 600 delays on American and more than 200 on Eagle operator Envoy Air by late afternoon, but Norton said many were unrelated to the computer issue.
The glitch prevented passengers at the three big hub airports from checking in for flights, and customers may have been unable to make reservations.
The incident occurred at an awkward time for American Airlines Group Inc., the world's biggest airline. In a month, the company plans to complete combining the reservations systems of American and its US Airways subsidiary and retire the US Airways brand.
Combining technology systems is a difficult feat that has tripped up other airlines, notably leading to several outages at United Airlines since it merged with Continental Airlines in 2010. United suffered two outages this summer, including one in July that the airline blamed on a faulty router.
American has made meticulous plans to avoid a similar fate. Among other moves, it is gradually switching US Airways flights to American over a 90-day period, and it will reduce flights to lighten the load on its network when it finishes merging the two reservations systems in mid-October. American officials said that Thursday's issues were unrelated to those measures.
In April, more than 50 American flights were delayed by a glitch in airport-map software loaded on pilots' iPads.
Shares of American Airlines Group rose 93 cents, or 2.2 percent, to close at $43.99. They have lost 18 percent this year.
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