United Airlines resumed operations on Wednesday night after a technology issue with its weight and balance computer system grounded all mainline flights, leading to over 1,000 delays and more than 40 cancellations, according to FlightAware.

The airline said the issue, which began shortly after 5 p.m. CT, was resolved within hours but warned of “residual delays.”

“We are working with customers to get them to their destinations after a technology disruption on Wednesday evening,” United said in a statement, per NBC News. “The underlying technology issue has been resolved, and, while we expect residual delays, our team is working to restore our normal operations.”

The outage prompted United to request ground stops at major hubs, including Newark, San Francisco, Chicago, Denver, and Houston, according to Federal Aviation Administration notices. United Express flights and airborne planes were unaffected, the airline said. United confirmed the issue was not a cyberattack and is covering meals and hotels for impacted customers.

U.S. Transportation Secretary Sean Duffy clarified that the issue was “specific to United’s operations” and “unrelated to the broader air traffic control system” after being briefed by United CEO Scott Kirby.

The FAA noted it was aware of the disruption affecting United’s operations.

Passengers reported significant inconvenience. One traveler in Denver told Fox News she was stuck on a United plane on the tarmac for nearly two hours, with no available gates and no update from the pilot on deboarding. She added that some planes had been delayed even longer.

United apologized on social media during the outage, stating, per Fox Business, “We’re aware of the system error at this time and are working on a fix to have you on your way as soon as possible. We understand that this disruption has caused frustrations during your travel and appreciate your continued patience.”

The airline emphasized, “Safety is our top priority, and we’ll work with our customers to get them to their destinations.”

Just last month, a significant IT outage led to a system-wide ground stop for Alaska Airlines and its regional subsidiary, Horizon Air, for about three hours, causing hundreds of residual cancellations and flight delays well into the next day.