United Airlines faced widespread disruptions Wednesday evening after a technology issue temporarily grounded all mainline flights. Hundreds of flights were delayed, and some were canceled, impacting passengers across the U.S. Key hubs like Newark, San Francisco, and Denver experienced ground stops. The airline confirmed the issue was not a cyberattack and that United Express flights were unaffected.

United Airlines temporarily grounded all of its mainline flights on Wednesday evening due to a technical issue, the carrier confirmed. Mainline operations refer to flights run directly by United using its aircraft and crew, including Boeing 737, 757, 767, 777, and 787 jets, along with Airbus A319 and A320 models, reports Fox News.

“Due to a technology issue, we are holding United mainline flights at their departure airports,” the airline stated. “We expect additional flight delays this evening as we work through this issue. Safety is our top priority, and we’ll work with our customers to get them to their destinations.”

In a recent update, the airlines stated that their systems are being restored, but delays are expected to continue throughout Wednesday night. So far, 827 flights have been delayed and 23 have been cancelled. The airlines have confirmed that it is not a cyberattack, reported ABC News. The ground stop does not impact United Express flights, and any flights already in the air will continue to their destinations as planned, as per the airlines.

Several FAA notices have confirmed that United Airlines requested ground stops at key hubs, including Newark, San Francisco, Denver, Houston, and Chicago. Angela Jeffers, a passenger whose flight from Nashville to Denver was delayed for two hours, said the pilot informed the passengers about a system outage. “We’re missing some numbers we need to take off so we can’t take off yet and don’t have a time estimate of when we’ll be able to,” Jeffers recalled the pilot as saying.