A power outage crippled air traffic at the world’s busiest airport for hours Sunday, stranding thousands of travelers and disrupting flights nationwide just as people begin the busy Christmas travel week.

Officials said Sunday night that the power outage at Atlanta’s Hartsfield-Jackson International Airport started with a fire in a Georgia Power underground electrical facility. The utility said the fire caused extensive damage, but power was restored to the entire airport around midnight.

The cause of the fire has not been determined.

Meanwhile, Delta Air Lines has canceled 300 flights Monday and was offering its customers traveling Sunday and Monday the opportunity to make a one-time change to their travel plans without penalty. “Limited commercial operations” were expected to resume at the airport Monday, the Federal Aviation Administration said in a statement during the height of the outage.

Three major airlines — American, Southwest and United — canceled arrivals and departures at the airport Sunday as a result of the power outage. Travelers were encouraged to check with their airlines before heading to the airport Monday.

There were reports that some planes sat on the tarmac for more than six hours waiting to move.

“First and most importantly, I want to express my sincere apologies to the thousands of passengers whose day has been disrupted in this manner,” Atlanta Mayor Kasim Reed said during a Sunday night news conference. “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 that shortly after 1 p.m., an electrical fire at one of the utility’s power substations knocked out the power.

Flightradar24, a flight-tracking website, said nearly 1,000 flights to and from the airport had been canceled Sunday.

At about 1:30 p.m., the airport tweeted that there was a power outage affecting “several areas” of the airport. Officials later elaborated to say a ground stop was in place, preventing Atlanta-bound flights from departing from other airports. Meanwhile, flights out of Atlanta were experiencing delays of more than 90 minutes, according to flight-tracking website FlightAware.

International flights were diverted to other airports, according to U.S. Customs and Border Protection.

The Federal Aviation Administration said in a statement that the power outage impacted the airport’s terminals, but the control tower did not lose power. “However, departures are delayed because airport equipment in the terminals is not working.”

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

The passenger who shot the footage, Natalie Seitz, 23, of New York, said her Delta flight to Cleveland was canceled Sunday afternoon. After initial anxiety inside Terminal B, she said, things had calmed down by about 4:30 p.m. — even as there was little indication of when power would be restored. She said employees where wheeling around a cart of bottled water.

In the terminal, “you walk around and hear people talking to each other and jokingly yelling at the PSA system when it repeats the same thing,” she said. “Most people are at this point taking this lightly, but some are aggravated.”

Like many travelers this weekend, Seitz is headed home for Christmas — to Cleveland, where her parents live. She said she was rebooked on a flight for later Sunday.

“We’ll see if the power comes on before then or not,” she said.