After hundreds of Southwest Airlines flights have been delayed by technical issues, the company is urging passengers to arrive 2 hours early and print boarding passes before coming to the airport.

Southwest has been using backup systems across the country to check-in passengers without printed or mobile boarding passes, but technological problems, which began Sunday morning, have become a continuing issue. About 450 of 3,600 Southwest Airlines flights scheduled for Sunday were delayed, according to the company.

The cause of technical difficulties has not been revealed, nor how long it will take to resolve. For now, there is "absolutely no indication" of hacking, according to Southwest spokesman Brad Hawkins.