DALLAS — Delta Air Lines said Tuesday that some computer systems are still working slowly more than a day after an outage crippled the airline and led to more than 1,500 canceled flights.

The system that the airline uses to check in and board passengers as well as dispatch its planes is still operating slowly, said Gil West, Delta’s chief operating officer.

West offered Delta’s most detailed explanation yet of what happened Monday to trigger the global computer outage: The Delta computers that control everything from reservations and boarding passes to crew and gate assignments toppled like a row of dominoes when one thing went wrong early Monday.

A power control module malfunctioned, causing a surge that cut off power to the airline’s main computer network. When that happens, the system is designed to switch in the blink of an eye to backup computer systems. On Monday, however, some of the backups did not kick in.

“Critical systems and network equipment didn’t switch over to backups,” West said in a written statement. “Other systems did. And now we’re seeing instability in these systems.”

Meanwhile, Delta passengers endured more canceled flights Tuesday, including a 12:30 p.m. flight to Atlanta from Portland International Jetport, as the carrier slogged through day two of its recovery from the meltdown. Hundreds more flights were delayed, including several in Portland, and by Tuesday evening Delta was posting delays of between 90 minutes and two hours. Other flights were on time.

Jetport Assistant Director Zach Sundquist said airport staff were pitching in to help Delta handle customer service issues while the airline worked to get caught up on its flight schedule.

“The biggest issue is trying to get the crews in the right locations for the flights,” Sundquist said.

By late afternoon, the airline said it had canceled 680 flights Tuesday as it moved planes and crews to “reset” its operation. Another 1,900 Delta flights had been delayed, according to tracking service FlightStats Inc.

In a video posted on the airline’s website, CEO Ed Bastian said Delta probably will have cancellations and delays Wednesday too, although he didn’t give numbers.

Confusion among passengers Monday was compounded as Delta’s flight-status updates crashed as well. Instead of staying home or poolside at a hotel until the airline could fix the mess, many passengers learned about the gridlock only after they reached the airport.

Then they were stuck.

Transportation Secretary Anthony Foxx’s office said the government was talking to Delta about technical issues surrounding the outage, but gave no specifics.

The Transportation Department said it also made sure Delta provided information about customer refunds on its website and that it was reviewing the consumer complaints that it had received.

Delta’s challenge was to find enough seats on planes during the busy summer vacation season to accommodate the tens of thousands of passengers whose flights were scrubbed.

Its hub in Atlanta was the epicenter of problem flights Tuesday, with lines that were much longer than the day before. Debbie McGarry left Switzerland on Monday and was still stuck Tuesday at the Atlanta airport, far from her Arizona home. Hopes of getting on a plane were raised and dashed overnight. By 3 a.m., passengers were getting irate.

“Some of the men were yelling,” she said. “I thought there might be a fistfight.”

Delta extended a travel-waiver policy to help stranded passengers rearrange their travel plans. It offered refunds and $200 in travel vouchers to people whose flights were canceled or delayed at least three hours. And Delta said it gave hotel vouchers to “several thousand” customers, including 2,000 in Atlanta.

Delta ranks as the third-largest in the world by number of passengers carried, with 138.8 million travelers last year, according to industry group IATA. It was narrowly beaten only by American Airlines and Southwest Airlines, with all of them flying mostly within the United States.

Airlines depend on huge, overlapping and complicated systems to operate flights, schedule crews and run ticketing, boarding, airport kiosks, websites and mobile phone apps. Even brief outages can snarl traffic and cause long delays.

Those types of problems have afflicted airlines in the U.S. and abroad. Last month, Southwest Airlines canceled more than 2,000 flights over four days after an outage that it blamed on a faulty network router. And United Airlines suffered a series of massive IT meltdowns after combining its technology systems with those of merger partner Continental Airlines.

Some passengers said they were shocked that computer glitches could cause such turmoil. Others took it in stride.

Ryan Shannon, scheduled to fly from Lexington, Kentucky to New York City, said passengers boarded, were asked to exit, waited about 90 minutes and then got back on the plane.

Once Delta cleared flights to take off, “We boarded and didn’t have any problems. There is always a delay, or weather, or something. I travel weekly, so I’m used to it,” Shannon said with a laugh.