(Bloomberg) -- Disruptions from a computer failure spilled into a second day for Delta Air Lines Inc., with cancellations on Tuesday morning reaching about 250 flights after one of the worst system faults experienced by the U.S. carrier.

A power outage at Delta’s Atlanta base at about 2:30 a.m. Monday interrupted computer operations, prompting the carrier to scrap about 1,000 flights the same day, with limited services resuming about six hours later. Chief Executive Officer Ed Bastian apologized after thousands of passengers were grounded as the second-biggest U.S. carrier struggled to restart its worldwide operations.

“I apologize for the challenges this is creating for you with your travel experience,” Bastian said in a video posted on the company’s website. “The Delta team is working very, very hard to restore and get these systems back as quickly as possible. It’s an all hands on deck effort.”

The disarray at Delta marked the second time in less than a month that a system failure forced mass flight cancellations at a large U.S. carrier. Southwest Airlines Co., the biggest U.S. discount airline, said a disruption July 20 would cost it “tens of millions” of dollars after more than 2,300 flights were canceled. Delta increased the number of flights dropped on Tuesday to about 250 in an e-mailed statement, from an earlier estimate of more than 100.

Whether Delta had an adequate backup system “is certainly a legitimate question,” said Bob Mann, president of aviation consultant R.W. Mann & Co. and a former airline executive. “Why didn’t the backup systems perform, or what was it about the systems that if they performed, it still didn’t allow the systems to communicate?”

Delta fell less than 1 percent to close at $37.44 in New York on Monday, leaving the shares down 26 percent this year.

Passengers Stranded

Among those stranded was Harold Jimenez, 29, who had to re-book his ticket for Tuesday to fly to Nashville, Tennessee, from New York’s LaGuardia Airport.

“I’ve been here since 6 a.m. I was supposed to take off at 8:40 a.m., and they told us it would be delayed until 11:30 a.m.,” Jimenez said. “But now they just canceled it.”

While the airline said it was giving snacks and beverages to passengers facing extended delays, passenger David Brennan at London’s Heathrow Airport said food vouchers ran out. He was scheduled to travel to San Francisco through Seattle on a 10-day vacation.

“I have two children. We have been here since 7:30 this morning,” he said. “It took two hours for them to tell us what had happened after the system went down. Then the pilot came and they said the flight was canceled because the crew can’t work more than 10 hours.”

Six flights were canceled at Tokyo’s Narita airport Monday, and some delays are expected Tuesday, Delta’s Japan-based spokeswoman Hiroko Okada said.

“We provided hotel accommodation to several hundred passengers,” she said. “Many remain at the airport, while some went home. Some booked hotels on their own.”

Waiving Fees

Delta waived change fees and fare differentials on ticket prices for passengers whose flights were canceled or delayed, according to a statement on its website. Passengers must begin their trip by Friday. The company also offered $200 travel vouchers to passengers whose flights were canceled or delayed by more than three hours.

“For those of you who’ve been inconvenienced and need to access and make changes to your travel plans, we’ve instituted system wide waivers,” Bastian said in the video.

The carrier booked inconvenienced travelers on other airlines in select cases but largely accommodated them on its own flights, spokesman Michael Thomas said.

Isolated Incident

No other airlines were affected, according to Hartsfield-Jackson Atlanta International Airport. The likely cause was a failure with Delta’s switch gear, which distributes energy throughout its building, said Craig Bell, a spokesman for electric utility Georgia Power.

“The power never stopped going, but when the system failed, power holds at the outside of the building until they can get it repaired,” he said.

Delta is investigating why some critical systems and network equipment didn’t switch over to its backup systems following the power loss, the carrier said in a statement. Flight-status displays, the website and some mobile and airport technology were disrupted, Delta said. The carrier didn’t estimate how long repairs would take.

‘Financial Burden’

“This will obviously mean a financial burden on Delta as it will have to arrange for alternate travel and hotel costs for passengers who are booked,” said Mark Martin, founder of Dubai-based Martin Consulting. The airline would probably use agreements with other carriers to arrange alternative travel for some customers, he said.

Last month at Southwest, computers were restored after about 12 hours but flights continued to be canceled or delayed for several days as the carrier worked to get crews and planes in the right locations.

U.S. carriers experienced technical disruptions to flights late last year and early in 2016, including a connectivity flaw at American Airlines that halted flights at its Chicago, Dallas and Miami hubs in September and a reservations-system glitch at Southwest in October.

A United Continental Holdings Inc. computer fault last summer lasted two hours and disrupted travel for thousands of fliers. It began with a router malfunctioning and prevented the carrier from ticketing passengers and dispatching crews.

Delta carried about 500,000 passengers daily during July, its busiest month, the airline said on its website.

--With assistance from Andrea Rothman Katerina Petroff Anurag Kotoky Dagney Pruner and Charlotte Ryan

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