ATLANTA, United States (AP) — The nation’s air-travel system struggled to get back on schedule and re-book stranded passengers Monday after a fire and blackout at the world’s busiest airport forced the cancellation of over 1,500 flights days before the start of the Christmas rush.

Travelers sat on the floor, slumped in chairs or stood in long lines at ticket counters a day after the underground blaze knocked out electricity and crippled Hartsfield-Jackson Atlanta International Airport for about 11 hours.

A spokesman for Delta, by far the biggest airline at the airport, said most of its delayed passengers were booked on other flights scheduled to leave Monday. Spokesman Michael Thomas said the airline should be “largely if not completely” back to normal by Tuesday, well before the huge travel weekend ahead of Christmas Day.

But no matter how fast Delta and other airlines move, it will take a few days to get the hundreds of thousands of grounded passengers to their final destinations, said Robert Mann, president of an airline consulting firm in Port Washington, New York. In rare cases, some passengers won’t arrive until Thursday, he said.

“There are just so few seats available during a peak holiday week, that’s just going to take a lot of flights with four or five seats apiece,” Mann said.

Southwest, the airport’s second-largest airline, said it was back on a normal schedule, but a spokesman could not say how long it would take to clear the backlog of stranded travelers.

American Airlines, which is much smaller, said that it, too, booked many of its passengers on new flights but that some will have to wait until later in the week to fly.

The fire broke out Sunday afternoon next to equipment for a backup system, causing that to fail, too. Power wasn’t fully restored until about midnight.

The control tower did not lose power because it has a separate electrical feed, and planes that were in the air and close to Atlanta when the blackout hit were allowed to land. Other incoming flights were diverted, and outgoing flights were halted.

Anthony Foxx, who was transportation secretary under President Barack Obama, was among many travelers stuck for hours in a plane on the tarmac. He blasted airport officials, saying the problem was “compounded by confusion and poor communication.”

“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!”

Georgia Power CEO Paul Bowers issued an apology and blamed the fire on a failure in a switch gear. He said the utility is considering a change in the setup of the main and backup systems to prevent a similar blackout.

Around noon Monday, stranded travelers sat on the floor, charging cellphones at the electrical outlets. An Atlanta city employee in a Santa hat gave out candy.

David and Lynn Carden, sitting in soft chairs in the airport’s atrium, left London early Sunday for Key West, Florida, but were diverted to Cincinnati because of the blackout. Delta got them a hotel room and put them on a Monday flight to Atlanta. From there, they awaited an afternoon flight to Florida.

“Delta has been pretty good,” David Carden said, counting themselves luckier than passengers who spent the night in an airport. “We don’t always get this kind of customer service in the UK”

College student Joe Ryan had planned to fly home to Chicago with his fiancee on Sunday on American after a four-day seminar in Atlanta. They spent Sunday night on a carpeted floor outside an elevator. He initially was told it could be Tuesday before he gets a flight home, but later he said he was able to get on a Monday flight.

Delta canceled about 1,000 flights Sunday and 400 more on Monday, in many cases because the pilots and airplanes were in the wrong places. To help clear the backlog, it added flights and found seats for some of its customers on other airlines.

Last spring, Delta was crippled by a storm in the South, and it took the airline five days — and about 4,000 canceled flights — before it fully recovered.

Thomas, the Delta spokesman, said that since then, the airline has put more flight crews on reserve and installed computer technology to quickly assemble properly rested crews.

Hartsfield-Jackson serves an average of 275,000 passengers a day. Nearly 2,500 planes arrive and depart each day.

Mann said the rebooking of passengers was probably complicated by the large number of inexperienced travelers this time of year.

“They’re more elderly, they’re more young people, they’re more infrequent travelers,” he said. “All these folks are going to require a lot of face time a lot of hand-holding.”