The White House initially said that there was no evidence of a cyberattack behind the outage that ruined travel plans for millions of passengers. American President Joe Biden said Wednesday morning that he directed the Department of Transportation to investigate.

The outage showed how dependent the world's largest economy is on air travel, and how much air travel depends on an antiquated computer system to generate alerts called NOTAMs _ or Notice to Air Missions _ to pilots and others.

The system was once telephone-based, with pilots calling dedicated flight service stations for the information, but has moved online.

The NOTAM system broke down late Tuesday and was not fixed until 9 a.m. Eastern on Wednesday, leading to about 1,200 flight cancelations and more than 7,800 delays by early afternoon on the East Coast, according to flight-tracking website FlightAware.

Even after the Federal Aviation Administration lifted the order grounding planes, the chaos was expected to linger. More than 21,000 flights were scheduled to take off in the U.S. Wednesday, mostly domestic trips, and about 1,840 international flights expected to fly to the U.S., according to aviation data firm Cirium.

Airports in Chicago, Los Angeles, New York and Atlanta were seeing between 30% and 40% of flights delayed.

“There was a systems issue overnight that led to a ground stop because of the way safety information was moving through the system,'' Transportation Secretary Pete Buttigieg said at a press conference. “That was resolved, which allowed the ground stop to be lifted at 9 this morning, but through the day we're going to see the effects of that rippling through the system.''

Longtime aviation insiders could not recall an outage of such magnitude caused by a technology breakdown. Some compared it to the nationwide shutdown of airspace after the terror attacks of September 2001.

“Periodically there have been local issues here or there, but this is pretty significant historically,'' said Tim Campbell, a former senior vice president of air operations at American Airlines and now a consultant in Minneapolis.

Campbell said there has long been concern about the FAA's technology, and not just the NOTAM system.

“So much of their systems are old mainframe systems that are generally reliable but they are out of date,'' he said.

According to 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 to keep departures flying overnight, but as daytime traffic picked up it overwhelmed the telephone backup system.

The FAA ordered all departing flights grounded early Wednesday morning, affecting all passenger and shipping flights.

Some medical flights could get clearance and the outage did not impact any military operations or mobility.

Flights for the U.S. military's Air Mobility Command, were not affected.

European flights into the U.S. appeared to be largely unaffected. Carriers from Ireland's Aer Lingus to Germany's Lufthansa said there was no impact on their schedules.