American Airlines was forced to ground all of its flights for several hours on Tuesday after a nationwide problem with its computer systems. By late afternoon, its computers were back up and its operation were slowly coming back to life.  More than 400 flights were canceled and scores more were delayed after a systemwide problem that started around midday and lasted until around 4:30 p.m. Eastern time. Still, the airline said it expected cancellations and delays to last through the rest of Tuesday as it tried to restore a normal schedule.  American said the problem was caused by an inability to gain access to its reservations system, called Sabre. The electronic system, often described as the brains of an airline, is responsible for bookings and reservations, but also manages a wide variety of functions related to flights, including printing boarding passes, online check-ins, ticketing, and tracking bags.  While the cause of the problem was not immediately clear, the airline emphasized that the flaw was not related to the bomb attacks in Boston on Monday. Safety officials at airports around the country were on high alert Tuesday. A suspicious package at La Guardia Airport in Queens, for instance, caused the main terminal to be evacuated briefly, causing an hourlong delay on some flights, officials said.  Sabre said American’s system problem did not come from its computer systems. Other airlines, including Southwest Airlines and JetBlue, use the reservation system and have not experienced any problems, said Nancy St. Pierre, a spokeswoman for Sabre.  Such nationwide breakdowns are rare but not unprecedented, particularly when airlines merge. United Airlines experienced similar problems last year when its reservation systems failed repeatedly as it merged them with those of Continental Airlines.  But while American Airlines, which is still reorganizing in bankruptcy, has agreed to merge with US Airways, the two airlines are nowhere close to integrating their systems because the planned merger still needs to clear regulatory hurdles.  American said that its network system had experienced “intermittent problems,” which led it to ground the fleet. The airline said it would waive fees for passengers who wanted to change their reservation on Tuesday and give refunds to those people who wished to cancel their flight.  At Dallas-Fort Worth International Airport, thousands of passengers were stranded and waiting for flights, some of which were being canceled. Some passengers described being stuck for long stretches on planes on the runway unable to take off or, having landed, initially unable to move to a gate.  Gate agents at Dallas were as uninformed as the passengers were, reduced to getting information from the airline’s Facebook page. “My best guess is y’all’s best guess,” one agent said.  Kelly Bixler, 34, a project coordinator for an architectural firm, had flown from Texarkana, Tex., to Dallas en route to Milwaukee on business but after waiting hours for her connection decided to rent a car and drive back to Texarkana. “There are so many people so upset,” she said. “They just want to go home. This is crazy.”  Matt Wyglendowski, 42, a sales manager for Hitachi trying to fly to Indianapolis, was angry that the airline was not more forthcoming about the event. “It’s ridiculous when you know more about what’s going on than the ticket people,” he said as he checked his smartphone. “I got the update on Facebook 20 minutes before they notified people.”  Mr. Wyglendowski said he had planned to drive from Indianapolis to his business meeting in Ann Arbor, Mich., to save money. “God knows when I’m going to get to Ann Arbor now,” he said.
The airline said its reservation system had failed, leading the carrier to halt flights across the country for several hours.