Domestic flights across the U.S. were temporarily grounded on the morning of January 11, after an I.T. failure in a critical aviation safety system. The Guardian reports that the ground stop was lifted at 9am January 11, with the FAA declaring that operations were “resuming gradually across the United States”, but travelers were still left facing another chaotic day of air travel following severe disruptions over the holiday period.

The Federal Aviation Administration (FAA) said that the system that alerts pilots and airlines about any hazards was not functioning. The breakdown led to more than 4,600 flights being delayed within, into, or out of the U.S., the flight tracking website FlightAware showed. It was not immediately clear if the outage was a factor.

“They don’t know what the cause is,” Joe Biden told reporters after speaking to the transportation secretary, Pete Buttigieg. “Aircraft can still land safely, just not take off right now. They don’t know what the cause of it is, they expect in a couple of hours they’ll have a good sense of what caused it and will respond at that time.”

The White House said there was no evidence of a cyber-attack but the causes of the I.T. failure would be investigated in full by the Department of Transportation.

International U.S.-bound flights were continuing to take off from Europe and elsewhere.