Delta Air Lines canceled roughly 1,000 flights Monday and delayed many more due to a power outage that affected its computer system, the airline said.
More than 3,000 of the nearly 6,000 flights scheduled for Monday actually got off the ground.
The airline had tweeted in mid-morning that although its system was "coming back online," there would be more delays and cancellations.
"I apologize for the challenges this has created for you with your travel experiences," CEO Ed Bastian said in a video statement addressing the day's troubles. "The Delta team is working very hard to restore and get these systems back as quickly as possible."
Delta began warning passengers of "large-scale" flight cancellations early Monday. The company tweeted that a computer outage at its Atlanta headquarters had delayed flights, noting that flights already in the air weren't affected.
"A power outage in Atlanta, which began at approximately 2:30 a.m. ET, has impacted Delta computer systems and operations worldwide, resulting in flight delays," Delta said in a statement. "Large-scale cancellations are expected today. All flights enroute are operating normally. We are aware that flight status systems, including airport screens, are 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."
Delta used Twitter to keep passengers up to date. The company tweeted that it was investigating the source of the outage.
WGCL-TV, the CBS affiliate in Atlanta, where Delta is based, reported that the problem was first uncovered when a flight was set to take off from San Francisco.
Update, 6:23 a.m. PT August 9: Adds statement from Delta CEO Ed Bastian.
CNET's Erin Carson contributed to this report. This story originally posted as "Delta warns of 'large-scale' flight cancellations" on CBSNews.com.