The culprit? A computer bug, reportedly.

By Daniel Roberts / Fortune
Updated: January 24, 2017
Courtesy of United

All United Airlines flights are currently grounded due to “automation issues,” according to an air traffic control system advisory notice. The airline has told CNBC the problem is affecting about 3,500 flights worldwide.

The notice says the stop is set to last until 9:15 a.m. ET, but it could be extended if the problem isn’t fixed. United suffered similar issues just last month. “United began delaying flights at approximately 8 a.m. CT to ensure aircraft departed with proper dispatching information,” the airline said at the time.

Fortune reached out to United for details or comment and did not hear back initially.

United Airlines merged with Continental Airlines in 2010; their parent company is now known as United Continental Holdings.

Update, 9:57 a.m.: United says, “We experienced a network connectivity issue this morning. We are working to resolve this and apologize to our customers for any inconvenience.”

Update, 10:02 a.m.: The ground stop has reportedly been lifted.

This article originally appeared on Fortune.

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