The Federal Aviation Administration is lifting a ground stop on flights across the U.S. following a computer outage early Wednesday that resulted in thousands of delays quickly cascading through the system at airports nationwide.

Earlier in the morning the FAA ordered all U.S. flights to delay departures until at least 9 a.m. Eastern. Due to heavy congestion, the FAA cleared flights to depart at Newark Liberty and Atlanta Hartsfield-Jackson airports.

The agency said that normal air traffic operations were resuming gradually across the U.S. following the outage.

More than 3,700 flights were delayed and more than 640 were canceled early in the day.

The FAA said that it continues to look into the cause of the initial problem.

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

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

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.

Some medical flights can get clearance and the outage is not impacting any military operations or mobility.

Early Wednesday, flights for the U.S. military's Air Mobility Command had not been impacted, said Air Force Col. Damien Pickart, a spokesman for Air Mobility Command is responsible for all the troop movement and supply flights, such as the C-17s that carry the president's motorcade vehicles when he travels, but also all the flights that transport troops from one base to another. Air Mobility Command was working with the FAA on the issue.

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 he's 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 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."

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

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.

Breakdowns in the NOTAM system appear to be rare.

"I don't ever remember the NOTAM system going down like this. I've been flying 53 years," said John Cox, a former airline pilot and now an aviation-safety consultant.

According FAA advisories, the NOTAM system failed at 8:28 p.m. Eastern on Tuesday preventing new or amended notices from being distributed to pilots. The FAA resorted to a telephone hotline in an effort to keep departures flying overnight, but as daytime traffic picked up it overwhelmed the telephone backup system.

Air Force spokeswoman Ann Stefanek said the U.S. military flights were not impacted because the military has its own NOTAMS system separate from the FAA system and the military's system was not affected by the outage.

European flights into the U.S. appeared to be largely unaffected.

Irish carrier Aer Lingus said services to the U.S. continue, and Dublin Airport's website showed that its flights to Newark, Boston, Chicago and Los Angeles were running on schedule.

"Aer Lingus plan to operate all transatlantic flights as scheduled today," the carrier said in a prepared statement. "We will continue to monitor but we do not anticipate any disruption to our services arising from the technical issue in the United States."

This is just the latest headache for travelers in the U.S. who faced flight cancellations over the holidays amid winter storms and a breakdown with staffing technology at Southwest Airlines. They also ran into long lines, lost baggage, and cancellations and delays over the summer as travel demand roared back from the COVID-19 pandemic and ran into staffing cutbacks at airports and airlines in the U.S. and Europe.

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