United Airlines temporarily grounded many mainline flights on Wednesday due to a technology issue, causing delays for thousands of passengers.

United Airlines experienced a temporary nationwide ground delay at key airports, including Denver, Newark, Houston, and Chicago, while operations for all other carriers proceeded normally.

The airline said safety is the top priority and warned of further delays. This outage is the latest in a series of tech-related disruptions in U.S. aviation.

Last month, Alaska Airlines faced a similar IT issue, and earlier this year, air traffic control failures at Newark raised safety concerns.

In January, a mid-air collision near Washington’s Reagan National Airport resulted in dozens of deaths.