Delta Air Lines cancelled around 427 flights on Monday after its computer systems crashed worldwide, stranding thousands of passengers on a busy travel day.

That number is likely to grow. More than 1,000 flights were delayed, according to flight tracking site FlightStats Inc.

About 11 hours into the outage, limited flights had resumed but widespread delays and cancellations were ongoing.

A power outage at an Atlanta facility at around 2:30 am. local time initiated a cascading meltdown, according to the airline, which is also based in Atlanta.

A spokesman for Georgia Power told The Associated Press that the company believes a failure of Delta equipment caused the airline’s power outage. He said no other customers lost power.

A Delta spokesman said he had no information on the report. Many passengers were frustrated that they received no notice of a global disruption, discovering that they were stranded only after making it through security and seeing other passengers sleeping on the floor.

It was unclear if the airline was even able to communicate due to its technical issues, and Delta said that there may be a lag issuing accurate flight status on the company website because of the outage.

Flights that were already in the air when the outage occurred continued to their destinations, but flights on the ground remained there.

Airlines depend on huge, overlapping and complicated systems to operate flights, schedule crews and run ticketing, boarding, airport kiosks, websites and mobile phone apps. Even brief outages can snarl traffic and cause long delays.

That has afflicted airlines in the US and abroad.

Last month, Southwest Airlines cancelled more than 2,000 flights over several days after an outage that it blamed on a faulty network router.

United has suffered a series of notorious delays since it merged with Continental as the technological systems of the two airlines clashed.

Lines for British Airways at some airports have grown longer as the carrier updates its systems. On Monday, in Richmond, Virginia, Delta gate agents were writing out boarding passes by hand. In Tokyo, a dot—matrix printer was resurrected to keep track of passengers on a flight to Shanghai.

Technology that appeared to be working sometimes issued bad information. Flight—status systems, including airport screens, incorrectly showed flights on time.

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