NEW YORK (AP) — A computer outage at the Federal Aviation Administration brought flights to a standstill across the US on Wednesday, with hundreds of delays quickly cascading through the system at airports nationwide.

The FAA ordered all US flights to delay departures until 9 am. Eastern, though airlines said they were aware of the situation and had already begun grounding flights.

At 8 am Eastern, there were more than 2,500 delayed flights within, into or out of the United States, according to the flight tracking website FlightAware, exceeding the number of all delayed flights on the previous day. More than 150 have been cancelled.

Those numbers are likely to grow.

More than 21,000 flights were scheduled to take off in the US today, mostly domestic trips, and about 1,840 international flights are expected to fly to the US, according to aviation data firm Cirium.

The White House said that there is no evidence of a cyberattack, but President Joe Biden directed the Department of Transportation to investigate the cause of the disruption.

President Joe Biden addressed the FAA issue Wednesday before leaving the White House. He said he had just been briefed by Transportation Secretary Pete Buttigieg, who told him they still had not identified what went wrong.

“I just spoke to Buttigieg. They don’t know what the cause is. But I was on the phone with him about 10 minutes,” Biden said. “I told him to report directly to me when they find out. Air traffic can still land safely, just not take off right now. We don’t know what the cause of it is.”

Buttigieg said in a tweet that he is in touch with the FAA and monitoring the situation.

Most delays were concentrated along the East Coast but were beginning to spread west. Inbound international flights into Miami International Airport continued to land, but all departures have been delayed since 6:30 a.m., said airport spokesman Greg Chin.

The FAA said it was working on restoring its Notice to Air Missions System.

“We are performing final validation checks and reloading the system now,” the FAA said. “Operations across the National Airspace System are affected.”

The agency said that some functions are beginning to come back online, but that “National Airspace System operations remain limited.”

United Airlines said that it had temporarily delayed all domestic flights and would issue an update once it learned more from the FAA. American Airlines said that it was closely monitoring the situation.

Julia Macpherson was on a United Airlines flight from Sydney to Los Angeles on Wednesday when she learned of possible delays.

The FAA is working to restore what is known as the Notice to Air Missions System.

There is a potential for widespread disruption because of the outage. All aircraft are required to route through the system, including commercial and military flights.

The FAA said that it would provide frequent updates as it made progress.