A power outage early Monday that halted Delta Air Lines flights worldwide was resolved just before 9 a.m., allowing some flights to resume travel.

The more than six-hour outage left thousands of customers scrambling to make their connecting flights and waiting for hours at departure gates to learn their flights’ fate. The Atlanta-based airline offered refunds to passengers whose flights were canceled or significantly delayed and waived change fees for others.

The airline has not yet announced what caused the power outage in Atlanta, and said it was still looking into the problem. A spokesperson for electric utility Georgia Power said the issue stemmed from equipment failure at a Delta site.

“We believe Delta Air Lines experienced an outage with their equipment. Other Georgia Power customers were not affected. We have staff on site attempting to assist them,” said John Kraft, a spokesperson for Georgia Power.

Airports saw large crowds Monday morning as stranded customers waited to see if their flights would depart, or if they could re-book on a different plane. Chris Paolino, a spokesperson for the Metropolitan Washington Airports Authority, said that as of 9:30 a.m. the crowds were starting to dissipate as flights resumed.

Cory Christensen was scheduled to leave Los Angeles at 12:30 a.m. Pacific time for a business trip in Washington. As of 9 a.m., he was still in Los Angeles, missed his connecting flight in Minneapolis and still was not sure he would make his business appointments on time.

He said he’s trying to find another flight. If he can’t book one soon, he might cancel the trip and get a refund.

“I’m tired,” Christensen said. “And hungry. Pretzels and soda isn’t much to get through the night on. It feels like I just pulled an all-nighter again from my college days.”

Delta advised customers to check the status of their flights before heading to the airport. At about 7 a.m., Delta said that some flight-status systems, including airport screens, were incorrectly showing flights on time.

“We apologize to customers who are affected by this issue, and our teams are working to resolve the problem as quickly as possible,” the airline wrote on its website.

Washington-area travelers flying Monday were affected by the outage. Delta operates out of Reagan National Airport, Dulles International Airport and Baltimore-Washington International Marshall Airport.

Delta is the latest airline to halt services because of a technical problem. Last month, Southwest Airlines canceled more than 2,000 flights nationwide after many passengers couldn’t check in or board their flights because of a computer glitch. Last year, United Airlines had to delay its planes for almost two hours — affecting nearly 5,000 flights — because of a computer glitch.

Staff writer Andrea Peterson contributed to this report.