(AP) -- Delta Air Lines canceled around 427 flights 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 cancelations were ongoing.

A power outage at an Atlanta facility at around 2:30 a.m. 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 U.S. and abroad.

Last month, Southwest Airlines canceled 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.

"Not only are their flights delayed, but in the case of Delta the website and other places are all saying that the flights are on time because the airline has been so crippled from a technical standpoint," said Daniel Baker, CEO of tracking service FlightAware.com.

Delta issued an apology to customers and said teams were attempting to fix the problem as quickly as possible.

Many passengers, like Bryan Kopsick, 20, from Richmond, were shocked that computer glitches could cause such turmoil.

"It does feel like the old days," Kopsick said. "Maybe they will let us smoke on the plane, and give us five-star meals in-flight too!"

In Las Vegas, stranded passengers were sleeping on the floor, covered in red blankets. When boarding finally began for a Minneapolis flight -- the first to take off -- a Delta worker urged people to find other travelers who had wandered away from the gate area, or who might be sleeping off the delays.

Word of the extensive breakdown began to spread after the airline used a Twitter account to notify customers that its IT systems were down "everywhere." Technological issues extended even to the company's website.

Tanzie Bodeen, 22, a software company intern from Beaverton, Oregon, left home at 4 a.m. to catch a flight from Minneapolis and learned about the delays only when she reached the airport and saw media trucks.

Bodeen said that passengers were taking the matter in stride. "It doesn't seem really hostile yet," she said.

The company said travelers will be entitled to a refund if the flight is cancelled or significantly delayed. Travelers on some routes can also make a one-time change to the ticket free of charge.

Yet many passengers still did not know where the rest of their day would be spent, and decisions on refunds would have to be made later.

At noon inside New York's LaGuardia Airport, Francesca Villardi still had no idea when her 11:50 a.m. flight to Fort Lauderdale, Florida, would depart.

The departure boards said her plane was leaving on time. She received different answers from three Delta employees, one of whom said she would be traveling to Cincinnati first.

"This is not organized at all," said the 51-year-old professional organizer from Pembroke Pines, Florida.

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Koenig reported from Dallas. Matt Small in Washington, Bree Fowler in Las Vegas, Joseph Pisani in New York, and Jeff Martin in Atlanta, contributed to this report.