NEW YORK (AP) — Thousands of flight delays and cancellations rippled across the U.S. early Wednesday after computer outage led to a grounding order for all departing aircraft by the Federal Aviation Administration.

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

Before commencing a flight, pilots are required to consult NOTAMs, or Notices to Air Missions, which list potential adverse impacts on flights, from runway construction to the potential for icing. The system used to be telephone-based, with pilots calling dedicated flight service stations for the information, but has now moved online.

While the White House initially said that there is no evidence of a cyberattack, President Joe Biden said “we don’t know” and told reporters that he’s directed the Department of Transportation to investigate the cause of the disruption.

The FBI did not immediately respond to a request seeking comment.

The stop order was lifted just before 9 a.m., but delays and cancellations are expected to snowball. Departure gates at major airports are filled with aircraft that had been ordered grounded for hours. More than 21,000 flights were scheduled to take off in the U.S. today, mostly domestic trips, and about 1,840 international flights expected to fly to the U.S., according to aviation data firm Cirium.

Nearly 5,000 flights were delayed and almost 900 were cancelled by around 10 a.m.

The stop order by the FAA impacted almost all flights of shippers and passenger airlines.

Some medical flights could get clearance and the outage did not impact any military operations or the mobility of U.S. defense forces.

Biden addressed the FAA issue Wednesday before leaving the White House to accompany his wife to a medical procedure at Walter Reed National Military Medical Center outside of Washington. 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.

Delays and cancellations that began on East Coast quickly spread west, with disruptions at almost all major U.S. airports.