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Lucy Nicholson/Reuters
Southwest Airlines warned passengers flying Monday to print boarding passes before going to the airport after technical problems delayed hundreds of flights across the country. The Dallas company said Sunday it was using backup systems nationwide to check in travelers who didn't printed their tickets or have access to mobile boarding passes, delaying around 450 of the 3,600 scheduled flights. Southwest officials didn't say what caused the problem or how much longer it was expected to last. The airline has advised passengers to arrive at least two hours before their scheduled departures.