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New York (CNN) — The Federal Aviation Administration lifted its order to halt all domestic flight departures across the United States Wednesday after it restored the system providing pilots with pre-flight safety notices. The overnight outage caused extensive disruption, and thousands of flights remain delayed across the country.
The agency put a ground stop order in place after its NOTAM -- or Notice to Air Missions -- system failed. The FAA lifted the order shortly before 9 a.m. ET, and the agency said normal air traffic operations were resuming across the country. It said it was still trying to determine the cause of the problem.
But airlines continued to delay or cancel flights because of ongoing congestion. An airline source familiar with the situation said airlines may implement ground delay programs, which could potentially lead to further timetable issues.
The FAA's website was still showing a ground delay at New York's LaGuardia Airport as of 10:50 a.m. ET. The site also showed a ground delay at Charlotte Douglas International Airport in North Carolina, one of American Airlines' largest hubs.
The Chicago Department of Aviation said ground stops at O'Hare and Midway had been lifted but "residual delays or cancellations" are likely.
Major US carriers including United Airlines, Delta and American Airlines all said they had grounded flights in response to the situation. United Airlines has issued a North America travel waiver in response to the outage.FlightAware, which tracks delays and cancellations, showed more than 6,100 flights to, from and within the United States as being delayed as of 10:50 a.m. ET, and more than 1,000 flights canceled so far.
Southwest, which canceled tens of thousands of flights after Christmas following a systemwide meltdown, has more cancellations and delays than the other airlines. About 9% of Southwest flights are canceled and 45% of flights are delayed.
The airline said mid-morning Wednesday that operations have resumed.
"As a result of the FAA's outage, we anticipate some schedule adjustments will be made throughout the day," Southwest said in a statement, encouraging travelers to check their flight status online or via the airline's app. Southwest has also issued a waiver allowing travelers to change their flights. Cause of outage still unclear
US President Joe Biden said there was no immediate information on what had caused the outage -- the second US aviation crisis in a matter of weeks. He said he had been briefed on the situation and was in touch with Transportation Secretary Pete Buttigieg.
"I just spoke with Buttigieg," he told reporters as he departed the White House. "They don't know what the cause is. But I was on the phone with him the last 10 minutes. I told them to report directly to me when they find out. Aircraft can still land safely, just not take off right now."
He continued, "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."
Asked whether it was a cyberattack, Biden said: "They don't know. They will find out."
There is "no evidence of foul play based on our discussions with DOT/FAA," a senior US official familiar with matter told CNN.
Earlier, White House Press Secretary Karine Jean-Pierre said in a statement that there was "no evidence of a cyberattack at this point," but that Biden had ordered a Department of Transportation investigation.
Buttigieg said via Twitter Wednesday morning that he had ordered an "after-action process to determine root causes and recommend next steps."
Calls came swiftly for aviation system upgrades.
"Today's FAA catastrophic system failure is a clear sign that America's transportation network desperately needs significant upgrades," said Geoff Freeman, president and CEO of the U.S. Travel Association.
"Americans deserve an end-to-end travel experience that is seamless and secure. And our nation's economy depends on a best-in-class air travel system."
International flights affected
International flights bound for the United States were continuing to take off from Amsterdam and Paris despite the situation. A Schiphol Airport spokesperson told CNN that "a workaround had been issued" and flights were still departing from Amsterdam.
No flights have been canceled from Paris' Charles de Gaulle airport, but delays were expected, according to the airport's press office. Frankfurt Airport also told CNN it had not been impacted.
A London Heathrow Airport spokesperson told CNN that they were "not aware of canceled flights and that flights to the US had left recently," however there were passenger reports of significant delays.
Shabnam Amini told CNN that she and other travelers had been sitting on board Americans Airlines flight 51 to Dallas for almost three hours at Heathrow because of the FAA outage.
She said they had been informed that there were delays but were still boarded onto the aircraft.
Commercial airline pilots use NOTAMS for real-time information on flight hazards and restrictions. The FAA stipulates NOTAMS are not to be relied on as a sole source of information, and so some flights may be able to satisfy safety requirements by using other data.
Wednesday's incident comes on the heels of another aviation crisis. A huge winter storm over the end-of-year holidays caused extensive disruption and helped trigger the Southwest Airlines meltdown that affected thousands of passengers.
CNN's Barry Neild, Paul P. Murphy, Betsy Kline, Livvy Doherty, Chris Isidore, Sean Lyngaas, Betsy Klein, Marnie Hunter and Stephanie Halasz contributed to this story.