(CNN) -- A power outage at the world's busiest airport left thousands of passengers stranded in dark terminals and in planes sitting on the tarmac, as a ground stop for Atlanta's Hartsfield-Jackson International disrupted air travel across the United States.
The outage affected all airport operations. Georgia Power said the cause has not yet been determined, though it may have involved a fire that caused extensive damage in a Georgia Power underground electrical facility. The fire impacted substations serving the airport.
Atlanta is the heart of the US air transport system, and the disruption led to flight delays and cancellations across the country. More than six hours into the outage, power was restored to one of seven concourses. The utility said it expected full power to be restored by midnight.
Here are the latest developments:
- Nearly 1,000 flights to and from Atlanta have been canceled, according to Flightradar24.
- Southwest, American and United airlines canceled operations in and out of Atlanta for the rest of Sunday.
- More than 900 Delta mainline and regional flights have been canceled and 48 flights were diverted to other airports.
- Flights headed to Atlanta are being held on the ground at their departure airport.
- Inbound flights to Atlanta are being diverted, US Customs and Border Protection said.
The blackout cut power in the terminals, leaving passengers stranded in the dark as they stood in line at gates and security checkpoints. Brittny Dettro said she was waiting to board a flight from Atlanta to Milwaukee when the power went out in Terminal B. She shot this image at 1:10 p.m. ET.
People used flashlights on their phones to see where they were going, said passenger Heather Kerwin, an Atlanta resident bound for New York.
"There were a few emergency lights on, but it was really dark -- felt totally apocalyptic," she said. "I decided to get the hell out of there."
Some passengers told CNN that airport and airline staff offered no updates as hours passed, leaving people scanning their phones and tablets for information. Stores stopped serving food and passengers were evacuated to alleviate crowding.
The outage left passengers sitting in planes on the tarmac for hours.
Jodi Green's Delta flight from the Bahamas landed at 1:15 p.m. ET Sunday. As of 7 p.m. she was still on the plane. Green said the pilot told passengers that other flights that had ran out of fuel were evacuated before theirs. Despite the circumstances, she said, order prevailed.
"People are calm, laughing, joking," she said. "I'm amazed I've been able to sit here and not lose my mind."
The ground stop led Southwest Airlines to cancel all operations in and out of Atlanta for the rest of the day, spokesman Brian Parrish said. Customers are being offered re-bookings without fare differences, he said.
United and American Airlines also suspended operations to and from Atlanta for the rest of Sunday. Delta, which has its headquarters and largest hub in Atlanta, canceled more than 900 Delta mainline and regional flights as a result of the ground stop.
Meanwhile, Atlanta Police urged people to stay away from the airport.
"We have no injuries, no one is harmed, everyone is safe, everything is orderly," Sgt. Warren Pickard said. "What we need the public to understand right away is that we need everyone to refrain from coming to the airport."
Three flights were canceled at the Quad City International Airport including two arrivals and one departure from Atlanta.
Members of the Patriot Guard Riders were there waiting to welcome military members coming home for the holidays.
Patriot Guard Rider member Eric Swanson says they see a few hundred service member fly in from the Atlanta airport every year.
He says they plan to make do with the delays.
“We're just going to be here from early morning until the last flight comes in at night. What comes, comes. If it comes in early, it comes in early. If it comes in late, it comes in late but we will be here,” says Swanson.