Federal officials said United crews had been unable to contact airline dispatchers through normal means.

“A software update caused a widespread slowdown in United’s technology systems,” United said in a statement. The airline said it was not a cybersecurity issue.

The Federal Aviation Administration, which runs the nation’s airspace, said United asked it to stop the airline’s departures nationwide. The FAA said the issue was limited to United and its subsidiaries.

The ground stop — it did not affect planes already in the air, United said — lasted a little more than an hour.

By late afternoon Tuesday on the East Coast, United had canceled only seven flights, well below its average of about 16 per day over the busy Labor Day weekend, according to figures from tracking service FlightAware.

However, more than 350 United flights were delayed — 13% of the carrier’s schedule, far more than rivals American, Delta and Southwest — on a day that many holiday vacationers were expected to fly home.

Southwest Airlines had a similar outage in April that grounded all departing flights for about an hour and caused more than 2,000 flights to be late.

The FAA caused all U.S. departures to be halted briefly in January when a system used to alert pilots to safety hazards failed. The agency blamed a contractor that it said accidentally deleted files while synchronizing the alert system and its backup.

Transportation Secretary Pete Buttigieg, who has criticized airlines for flight problems and other issues over the past year, posted Tuesday that the FAA was “receiving more information about the cause and scope of the issue, and DOT will make sure (United) meets its obligations to affected passengers.”

The FAA is part of Buttigieg’s Department of Transportation.

Shares of Chicago-based United Airlines Holdings Inc. fell on news of the ground stop and closed down 2.5%.