ATLANTA (AP) — Ticketing and baggage areas were a bit backed up, but security lines were moving quickly in Atlanta on Monday morning as the world's busiest airport recovered from a crippling power outage.

Thousands of people remained stranded a day after the outage at Hartsfield-Jackson Atlanta International Airport, where more than 1,000 flights were grounded just days before the start of the Christmas travel rush.

A sudden power outage that Georgia Power said was caused by a fire in an underground electrical facility brought the airport to a standstill Sunday around 1 p.m.

All outgoing flights were halted, and arriving planes were held on the ground at their point of departure. International flights were diverted, officials said.

Delta Air Lines, with its biggest hub in Atlanta, was hardest hit. By Sunday evening, Delta had already canceled nearly 900 flights and another 300 Monday, nearly all of them in Atlanta, according to tracking service FlightAware.com.

Robert Mann, an aviation consultant and former American Airlines executive, said it likely will be Tuesday before Delta's operations in Atlanta return to normal, and for passengers "it could be most of the week" because there aren't many open seats on other flights in the last week before Christmas.

One bit of good news, according to Mann: Delta has more spare planes and available crews in Atlanta than anywhere else, which should help it to recover.

Delta customers flying to or from Atlanta can make a one-time change to travel plans without incurring a $200 change fee. The airline also encouraged travelers not to pick up their bags Monday because of anticipated congestion at the airport.

Still, when flights at Atlanta were grounded for most of one day last spring, it took Delta five days — and about 4,000 canceled flights — before it fully recovered.

Like Sunday's outage, that April storm hit Delta's largest hub at a busy travel time when there weren't many empty seats to accommodate customers from canceled flights. At the time, CEO Ed Bastian vowed Delta would make "significant improvements" to its system for scheduling and tracking aircraft crews to recover more quickly from disruptions.

Other airlines also canceled flights for the rest of Sunday. American Airlines canceled 24 departures and an equal number of arrivals, said spokesman Ross Feinstein. The airline also diverted three planes that were headed to Atlanta when the outage struck, sending them instead to Dallas, Nashville and back to Philadelphia.

The city of Atlanta provided shuttle service to the Georgia Convention Center on Sunday for travelers needing a place to stay.

Delta passenger Emilia Duca, 32, was on her way to Wisconsin from Bogota, Colombia, when she got stuck in Atlanta. She said police made passengers who were in the baggage claim area move to a higher floor. She said restaurants and shops were closed. Vending machines weren't working.

"A lot of people are arriving, and no one is going out. No one is saying anything official. We are stuck here," she said. "It's a nightmare."

Some passengers said there was a lack of information from airport officials and little help from first responders to get the disabled and the elderly through the airport without the use of escalators and elevators.

"They had these elderly people, handicapped people lined up in wheelchairs," said stranded passenger Rutia Curry. "The people were helpless, they can't get down the stairs. It was just a nightmare."

Passenger James Beatty said there was no real method for evacuation.

"I mean there was 40 or 50 people per the terminal area that were confined to wheelchairs and some that couldn't get through the airport very well, some of them actually couldn't walk and there was no plan at all to get them out of here without any power."

Beatty said passengers carried those who used wheelchairs down stairs.

According to a Georgia Power statement, the utility believes a piece of equipment in an underground electrical facility may have failed, causing the fire. The fire was next to equipment for a backup system, causing that to also fail.

"No personnel or passengers were in danger at any time," the statement said.

No areas outside of the airport were affected by the power loss. The utility said there are "many redundant systems in place" to ensure the power supply to the airport and that such outages at the airport "are very rare."

The utility apologized for the inconvenience and thanked people for their patience in a tweet Monday morning that included a photo of workers in a tunnel. All essential services were restored and an investigation into the cause of the outage was ongoing, the tweet said.

Anthony Foxx, who served as U.S. transportation secretary under former President Barack Obama, tweeted that he was among the many travelers stuck for hours on a plane on the tarmac.

"Total and abject failure here at ATL Airport today," he tweeted, adding that there was "no excuse for lack of workable redundant power source. NONE!"

In another tweet, Foxx said it seemed like the problem was "compounded by confusion and poor communication."

Once he was off the plane, Foxx tweeted that he hoped to rent a car to drive to Charlotte, North Carolina, to catch a flight Monday morning.

Sara Melillo and her husband, Greg Presto, were traveling from Kenya, where they live, to Pittsburgh to spend Christmas with his family when they were stuck on the tarmac for six hours. The couple had made stops in Nairobi and Amsterdam and landed shortly after the lights went out in Atlanta.

Melillo said the pilot didn't have a lot of information for the travelers but the plane had air conditioning and attendants offered water and juice a few times. She described the Delta terminal as "big chaos" with not enough customer service for the hundreds of people trying to find a flight to their next destination and a place to sleep for the night.

In a Monday morning email, Melillo told The Associated Press that the couple was scheduled to leave for Pittsburgh around 11 a.m. The Delta check-in process was organized and lines were moving, she wrote. Airport workers were handing out doughnuts and other breakfast foods.

At Southwest Airlines, about 70 Atlanta departures out of 120 scheduled for Sunday were canceled, an airline spokesman said in an email. United Airlines and JetBlue Airways were among carriers reporting delays or cancellations.

American Airlines reported only a handful of diversions and cancellations because the carrier does not use Atlanta as a hub, airline spokeswoman Alexis Aran Coello.

Hartsfield-Jackson, which serves 104 million passengers a year, is the world's busiest airport, a distinction it has held since 1998.

The airport serves an average of 275,000 passengers daily, according to its website. Nearly 2,500 planes arrive and depart each day.

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AP Airlines Writer David Koenig in Dallas and Associated Press writers Kate Brumback and Robert Ray in Atlanta contributed to this report.