An outage with a federal pilot-alert system cascaded into a nationwide logjam at U.S. airports Wednesday, snarling thousands of flights and temporarily stranding travelers across the country.

The breakdown of the Federal Aviation Administration’s Notice to Air Missions system, which provides safety information to pilots, led the agency to issue a nationwide “ground stop” that halted domestic departures for nearly two hours, before flights were permitted to resume shortly after 9 a.m. ET.