Flights in the United States were gradually beginning to resume departures on Wednesday after a system under the country’s Federal Aviation Administration stopped functioning for over two hours.

Nearly 5,000 flights were delayed and almost 900 were cancelled by around 10 am Eastern Standard Time after the Notice to Air Missions system stopped working, the Associated Press reported. The system alerts pilots to hazards and changes to airport facilities.

“The ground stop has been lifted,” the Federal Aviation Administration said in a tweet. “We continue to look into the cause of the initial problem.”

The White House said there was no evidence of a cyberattack so far.

President Joe Biden said that he has directed the Department of Transportation to investigate the outage.

“They don’t know what the cause is,” Biden told reporters before flight operations resumed. “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.”

Meanwhile, airports in Philadelphia, Tampa, Florida, Austin and Texas also advised passengers to check with their airlines for information about the delays.