A disruption to a system housing United Airlines’ flight information that caused more than 1,000 delays has been resolved, but delays continued into Thursday.

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. While residual delays were expected, United said its team was working to restore normal operations.

About 35% of all the airline’s flights were delayed and another 7% were canceled Wednesday, according to FlightAware, a website that tracks flight disruptions. By Thursday morning, 4% of United flights for the day were delayed and 4% were canceled.

An alert on the Federal Aviation Administration website Wednesday 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.

The airline’s travel alert page noted that a technology disruption might impact flights to or from several U.S. airports from Thursday to Sunday including Denver, Newark, Washington, Houston, New York, Los Angeles, Orlando, Chicago, San Francisco, Honolulu and Guam and some in Europe, such as London, Frankfurt and Munich.

“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. United said it was treating it as a controllable delay, meaning it paid customer expenses such as hotels when applicable.

The FAA said in a statement Thursday that it was aware of the disruption to United operations, noting that delays may continue as the airline recovers.

“We’ve offered full support to help address their flight backlog and remain in close contact with United,” the FAA said.

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.”