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

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