A global computer system failure at Delta Air Lines left hundreds of passengers stranded for up to 12 hours at the Portland International Jetport on Monday.

Twelve hours after a power outage knocked out its computer systems worldwide, Delta Air Lines was still struggling to resume normal operations and clear backlogs of passengers stranded by canceled flights.

By early afternoon, Delta said it had canceled 451 flights around the globe. Tracking service FlightStats Inc. counted 2,000 delayed flights – about one-third of the airline’s entire schedule.

Delta representatives said the airline was investigating the cause of the meltdown. They declined to describe whether the airline’s information-technology system had enough built-in redundancies to recover quickly from a hiccup such as a power outage.

Many passengers were frustrated that they received no notice of a global disruption, discovering that they were stranded only after making it through security and seeing other passengers sleeping on the floor. Delta said the outage caused a lag in posting accurate flight-status information on its website.

All Delta flights to and from the Portland International Jetport had been delayed or canceled as of 5 p.m. Monday, affecting an estimated 1,750 scheduled passenger trips. The longest delays were for flights headed to Atlanta, Delta’s hub. The first departure from Portland to Atlanta was supposed to leave at 5:50 a.m. As of 5 p.m., it had yet to leave and was tentatively scheduled to depart at 5:45 p.m.

Jetport Director Paul Bradbury said that while he was not in a position to say why each individual flight was delayed or canceled, the general issue was that initial flight delays caused a ripple effect throughout the day, similar to when there is a major weather event.

In some cases, flight crews had “timed out” from working or being on-call too long, and there was difficulty finding other crews to replace them, he said. In other cases, earlier delayed flights were late returning to Portland to carry the next load of passengers.

“These are the same aircraft, that is the problem,” Bradbury said. “You need one to leave before they can return (and pick up more passengers).”

Bradbury said Delta set up customer service centers at two gates, as well as the ticket counter, to assist stranded passengers. Delta also provided free snacks and beverages to customers during the delay.

Although Delta allowed passengers to switch their flights to other airlines Monday at no additional charge, Bradbury said there simply wasn’t room on most flights to accommodate stranded Delta passengers.

“In the summer, our load factors are very high in Portland,” he said. “There’s not enough room to absorb all the canceled flights.”

WAITING AND WONDERING

Delta said that almost 1,700 of its scheduled 6,000 flights had operated by midafternoon. The airline posted a video apology by CEO Ed Bastian, who stood in the airline’s technology center and assured customers that employees were working hard to resume normal operations.

A power outage at an Atlanta facility about 2:30 a.m. local time initiated a cascading meltdown, according to the airline, which is based in Atlanta.

A spokesman for Georgia Power said the company believes a failure of Delta equipment caused the airline’s power outage. He said no other customers lost power.

Delta spokesman Eric O’Brien said he had no information on the report and that the airline was still investigating.

Joshua Silver, a cyber security expert and shareholder at Bernstein Shur law firm in Portland, said it is surprising that a large airline such as Delta could find itself completely paralyzed by a computer system failure. He noted that other airlines, including Southwest Airlines and JetBlue, have suffered similar problems recently.

“Airlines that have experienced these sorts of problems almost certainly have robust disaster recovery (and) continuity of business systems in place that are intended to avert a system failure like this one,” Silver said via email. “My best guess is that airline computer systems are extremely complex and contain many layers of systems. It is possible that when one of those layers fails, it affects some of the others.”

Silver said that if enough layers of a complex computer system are affected, it brings the entire system down. Some failure in the Delta computer systems must have prevented the disaster recovery systems from working properly, he said.

“I’m sure that was a little irritating for Delta given that they have probably spent many millions of dollars building out their disaster recovery systems,” Silver said.

Flights that were already in the air when the outage occurred continued to their destinations, but flights on the ground remained there, including all of the airline’s flights to and from Portland.

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.

Daniel Baker, CEO of tracking service FlightAware.com, told the Associated Press that after years of rapid consolidation in the airline business, these computer systems may be a hodgepodge of parts of varying ages and from different merger partners. The systems are also being worked harder, with new fees and options for passengers, and more transactions – Delta’s traffic has nearly doubled in the past decade.

“These old legacy systems are operating much larger airlines that are being accessed in many, many more ways,” Baker said. “It has really been taxing.”

That has afflicted airlines in the United States and abroad.

Last month, Southwest Airlines canceled more than 2,000 flights over several days after an outage that it blamed on a faulty network router.

United Airlines suffered a series of massive IT meltdowns after combining its technology systems with those of merger partner Continental Airlines.

Lines for British Airways at some airports have grown longer as the carrier updates its systems.

On Monday in Richmond, Virginia, Delta gate agents were writing out boarding passes by hand. In Tokyo, a dot-matrix printer was resurrected to keep track of passengers on a flight to Shanghai.

“Not only are their flights delayed, but in the case of Delta the website and other places are all saying that the flights are on time because the airline has been so crippled from a technical standpoint,” Baker said.

Many passengers, like Bryan Kopsick, 20, from Richmond, were shocked that computer glitches could cause such turmoil.

“It does feel like the old days,” Kopsick told the AP. “Maybe they will let us smoke on the plane, and give us five-star meals in-flight too.”

The company said travelers will be entitled to a refund if the flight is canceled or significantly delayed. Travelers on some routes can also make a one-time change to the ticket free of charge.

Yet many passengers still did not know where the rest of their day would be spent, and decisions on refunds would have to be made later.