Power was restored to the world’s busiest airport Monday, but thousands of people were still stranded in Atlanta or faced disruptions in Atlanta-bound flights as the problems spread for the pre-Christmas travel chaos.

The outage at Atlanta’s Hartsfield-Jackson International Airport hit Sunday afternoon, triggered by a fire in an underground electrical facility that caused heavy damage.

While the investigation into the cause of the fire is ongoing, officials with Georgia Power said they do not suspect sabotage. Instead, they think a switch gear in the facility may have failed and caused the blaze. Systems that would have provided backup power also failed, they said, because the circuit cables and switching mechanisms were housed in an area adjacent to and also damaged by the fire.

Cybersecurity officials with the Department of Homeland Security said Monday that they do not think the outage was the result of “an attack or other nefarious act.”

Investigators did not find “a cyber nexus to this event,” DHS officials said, adding that the power failure did not cause any lasting damage to the airport’s communication systems.

Power was restored about 12 hours after the initial outage, but by then, more than 1,000 flights had been grounded.

Even with the resumption of air service, delays remain and could take days to clear up. Hartsfield-Jackson is the world’s busiest airport, with about 2,500 daily departures and arrivals, according to the airport.

The Federal Aviation Administration said in a statement that while the power outage affected the airport’s terminals, the airport’s control tower did not lose power. Even so, the agency instituted a temporary ground stop, halting air traffic into and out of the airport.

In a news conference Sunday evening, Atlanta Mayor Kasim Reed apologized to the thousands of passengers stranded by the outage. Others were stuck for hours on planes as they sat on the tarmac.

“We certainly understand that the outage has caused frustration and anger, and we’re doing everything that we can to get folks back home right away,” Reed said.

In one video, passengers at an airport terminal were seen sitting in the dark as they awaited instructions over the PA system.

At Reagan National Airport, about 300 flights were canceled Monday.

In Atlanta, as flight operations resumed, there were few people in lines at the airport early Monday.

Travelers were advised to contact their airline before heading to the airport. Delta is allowing customers to make a “one-time” change without penalty.

Delta canceled more than 300 flights for Monday morning but in a Twitter message said it expected flights in Atlanta to get back to normal by Monday afternoon.

The Federal Aviation Administration warned at the height of the outage that travelers should expect “limited commercial operations” for Monday at the airport.