(AP) – A technology issue prompted United Airlines to ground planes at major U.S. airports Wednesday and more than 1,000 flights were delayed.

The impacted system, called Unimatic, houses flight information that is fed to other systems including those that calculate weight and balance and track flight times, according to United. It’s not clear what caused the problem, which was resolved late Wednesday, though some service disruptions continued into Thursday.

An alert on the Federal Aviation Administration website said all United flights destined for Chicago were halted at their departing airports. Flights to United hubs at Denver, Newark, Houston and San Francisco airports also were affected.

“Safety is our top priority, and we’ll work with our customers to get them to their destinations,” an emailed statement from the Chicago based-airline said.

The system outage, as the company described it, lasted several hours, the statement said. It wasn’t related to recent concerns about airline industry cybersecurity.

About 35% of all the airline’s flights were delayed and another 6% were canceled Wednesday, according to FlightAware, a website that tracks flight disruptions.

Johan Kotze was one of the many travelers frustrated by the disruptions. He was at Louis Armstrong New Orleans International Airport to begin a journey to the Indian Ocean island nation of Mauritius for vacation.

Getting caught up in flight delays for him likely meant he would miss connecting flights along the way and would have to rebook not only the flights but a car and accommodations as well.

“It’s not very nice,” he said of the experience.

United Airlines apologized on social media and said in some cases it would pay for hotel and other expenses incurred by travelers because of the delays.

“Hey there, we apologize for the travel disruption today,” the airline told a customer on the social platform X. “Our teams are working to resolve the outage as quickly as possible. Thank you for your patience.”