ATLANTA -- A power outage grounded flights Sunday at Atlanta's Hartsfield-Jackson Airport, but the cause of the outage is not known. The outage left thousands of passengers sitting in the dark on a busy day for holiday travel.

Officials from the Georgia Power utility issued a statement Sunday night that says the company "expected to have power restored to the airport by midnight tonight."

As of late Sunday, officials from the Atlanta Airport say electricity has been restored to the atrium and several concourses (including T, A and B) at Atlanta's international airport.

Power is back ON in Atrium and Concourses T, A and B! — Atlanta Airport (@ATLairport) December 18, 2017

Georgia Power officials previously said they restored power to concourse F earlier Sunday.

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The power utility mentioned that the cause of the power outage "may have involved a fire which caused extensive damage in a Georgia Power underground electrical facility. The fire was safely extinguished by fire crews ..."

CBS News transportation correspondent Kris Van Cleave reports that the airport has backup generators that allow for minimal operation but the outage is impacting the airport's ability to operate substantially.

Airport spokesperson Reese McCranie told The Associated Press the outage occurred early Sunday afternoon. He said all airport operations are being affected and that outgoing flights were halted.

A view inside Atlanta's Hartsfield-Jackson Airport on Sunday, Dec. 17, 2017. Brian Reinert/Twitter

McCranie says some incoming flights are being diverted to other airports in the region.

The airport said on its Facebook page that departures were delayed because airport equipment in the terminals is not working.

The Federal Aviation Administration, meanwhile, has implemented a "ground stop" for flights headed to the airport. A ground stop means that flights headed to Atlanta are held on the ground at their departure airport.

The AP reports that Delta -- with its biggest hub operation in Atlanta -- will be hardest hit. By evening, Delta had already canceled almost 800 Sunday flights and another 250 on Monday, nearly all of them in Atlanta, according to tracking service FlightAware.com.

Mayor Kasim Reed tweeted Sunday night that all passengers had been safely deplaned, AP reports.

In addition, the City of Atlanta said on its Twitter page that it would provide shuttle service to the Georgia Convention Center for travelers in need of a place to stay and Chick-fil-A would be provided.