Destiny Easley missed her flight because of the long lines at the Delta Air Lines ticket counter on Monday at the Atlanta airport. (John Spink/AP)

Officials at Georgia’s leading power provider faced tough questions Monday, a day after a massive power outage grounded travel at the world’s busiest airport for nearly 11 hours at the start of one of the biggest travel weeks of the year.

Georgia Power officials said the investigation continues into the cause of the fire-related outage at Hartsfield-Jackson Atlanta International Airport. However, they said a piece of the utility’s switchgear in an underground electrical facility may have failed and started the blaze. A switchgear helps to manage the flow of power.

The utility had backup equipment, officials said, but it was in an adjacent room and was also damaged in the fire.

“We are doing what we can to make sure this never happens again,” Georgia Power spokesman Craig Bell said. “We truly apologize for the inconvenience.”

Bell said the utility’s systems include multiple backups that continue switching until a power source is found. He characterized the outage Sunday as an “anomaly.”

Stranded passengers wait near the baggage claim at Hartsfield-Jackson Atlanta International Airport on Monday. (Jessica Mcgowan/Getty Images)

Fires “almost never happen in one of our underground facilities,” he said. “Mechanical breakdowns can happen and the system continues to switch until it finds a source, but when you add fire — that was the wild card.”

The Department of Homeland Security said federal officials do not think the outage was the result of an attack or other “nefarious act.” Cybersecurity investigators with the federal agency said they did not find a “cyber nexus to this event.” They added that the power failure did not cause lasting damage to the airport’s communications assets systems.

Meanwhile, airlines were focused on playing catch-up — particularly Delta Air Lines, which is responsible for nearly 80 percent of the flights at Hartsfield-Jackson, a hub in the nation’s air transportation network.

Delta brought in nearly 200 extra employees to help manage the crush of passengers and offered one-time fee waivers so affected travelers could rebook their tickets. Other carriers, including American Airlines, JetBlue Airways and United Airlines, quickly followed suit.

Television footage showed long lines of people waiting at ticket counters and in security screening lines.

Delta started Monday with 390 flight cancellations but by afternoon, Gil West, the airline’s chief operating officer, said, things were nearly back to normal. The airline worked to clear the backlog of passengers by adding flights and finding seats on other airlines.

Indeed, Monday evening, the flight tracking website listed virtually no flight cancellations for Hartsfield-Jackson on Tuesday.

A traveler scans the flight status display in Atlanta. (Jessica Mcgowan/Getty Images)

The quick recovery is important. Officials with the Transportation Security Administration expect Thursday and Friday to be the busiest pre-Christmas travel days, with more than 2.1 million passengers and crew members passing through screening checkpoints.

The outage hit about 1 p.m. Sunday, plunging the airport into darkness, grounding nearly 1,000 flights and leaving 30,000 people stranded. International flights were diverted to other airports. Planes sat on the tarmac for more than six hours.

The terminal lost power but the control tower did not, according to the Federal Aviation Administration. However, tower operations were hampered because airport equipment in the terminal, including computers, wasn’t working.

Videos posted on Twitter showed passengers using the lights on their cellphones to make their way through darkened concourses. Other photos shared on Twitter showed TSA officers carrying wheelchairs up stalled escalators.

Natalie Seitz, 23, of New York posted a video on Twitter about 90 minutes after the airport announced that it had lost power. "An emergency has been reported in the building," a mechanical voice kept saying, before instructing passengers to "stand by for further instructions," as emergency lights flashed.

"You walk around and hear people talking to each other and jokingly yelling at the PA system when it repeats the same thing," Seitz said.

Despite the criticism, FAA officials said the airport followed its emergency plan — a document that was reviewed and approved by the agency.

“We will work closely with the airport in its post-incident investigation and further assess if the airport met its AEP,” the agency said a statement.

Atlanta Mayor Kasim Reed (D) responded to questions about the airport’s power supply during a news conference Sunday night. Specifically, why was there no redundancy?

“The straight answer to that question is: We absolutely do” have a redundant power supply, Reed said. “But because of the intensity of the fire, the switch that accesses the redundant system was damaged, which caused damage to two systems rather than one.”

Georgia Power officials said that power was restored for all “essential” airport activities about 11:45 p.m., enabling flights to resume.

Officials said firefighters arrived about four minutes after the fire was first reported. It took about two hours to extinguish the blaze. Georgia Power officials said crews then had to wait for hazardous fumes and toxic smoke to clear before they could enter the tunnels and begin repair work.

In an appearance on "Good Morning America," on Monday, Georgia Power chief executive Paul Bowers said the investigation is expected to take the remainder of this week.

“We have our investigators in the tunnel,” he said.

Bowers said the utility will work with the airport to prevent a repeat of such an event. Among the strategies the company may consider: encasing the cables in concrete or putting the cables in separate areas.

“Our focus is on reliability and making sure this never happens again,” he said.

Marcia Hale, president of Building America’s Future, a bipartisan coalition of elected officials who advocate for infrastructure investment, said what happened in Atlanta is a reminder about the importance of investing in systems that keep people moving.

“I do think Atlanta is a very unusual situation, but if what we’re being told is true and they had the backup right next to primary — we’ll I’m sure after action report will say that probably should not have happened. It’s classic that the redundancy didn’t work there. It just so highlights the need to modernize.”

Nick Miroff and Faiz Siddiqui contributed to this report, and information from the Associated Press was used in this report.