In a morning of chaos on Wednesday, the Federal Aviation Administration lifted a ground stop order for all flights just before 9 a.m. ET, after fixing a system failure caused by an overnight outage.

The NOTAM (Notice to Air Missions) system—which provides essential aviation information such as airspace obstacles and closed runways to pilots and other airport personnel—stopped processing information early Wednesday. As a result, the FAA ordered airlines to temporarily pause all U.S. domestic departures. The stop has caused widespread disruption, with Flightaware reporting more than 13,000 flight delays and 2,000 cancellations today.

Addressing the technical problem, the transportation agency tweeted: “Normal air traffic operations are resuming gradually across the U.S. following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews.”

“We continue to look into the cause of the initial problem.”

U.S. Transport Secretary Pete Buttigieg issued a statement on Twitter, noting that he had been in touch with the FAA this morning to discuss the outage, before echoing the agency’s reassurance. “FAA is working to resolve this issue swiftly and safely so that air traffic can resume normal operations, and will continue to provide updates,” the statement said.

Buttigieg briefed President Joe Biden on the system failure, said White House Press Secretary Karine Jean-Pierre, adding that there is no current evidence of a cyberattack but the Department of Transport will conduct a “full investigation” into the causes.

The FAA has been operating without a leader since last March when the previous administrator, Stephen Dickson, left before the end of his term. President Biden’s nominee, Phillip Washington, has yet to receive a confirmation hearing and has faced scrutiny due to his limited experience and involvement in a public corruption investigation, according to the New York Times.

What are NOTAMS and why are they important?

The Notice to Air Missions System updates pilots on “closed runways, equipment outages, and other potential hazards” on a flight route or at a location that could affect a journey, the FAA explained. The notices offer rolling information on the state of flying for pilots that may not be known sufficiently in advance of a flight’s departure.

Ciricum, an aviation analytics website, noted that there are as many as eight types of NOTAMS in aviation, advising on issues such as airport restrictions and weather, only some are mandatory for pilots to address, others are more advisory in nature. Pilots are expected to review NOTAMS ahead of takeoff and they are reportedly part of the pilot’s so-called flight bag—documents pilots must have before an aircraft can undertake its intended route.

Macheras told TIME that NOTAMS are “essential for the safe continuation of global air travel” and these notices keep the world’s aviation sector, as well as all crew and personnel concerned with flight operations, “up to speed with latest air travel related directives, operational updates, security, weather and warnings.”

How fliers reacted to the delays?

Angry passengers facing delays and cancellations took to social media to complain. One Facebook user wrote, “Heading to Hollywood, Florida for the Seminole Main Event Poker tournament. Been at airport since 4:30 am for a 6 am flight. Now we’re being told that all US flights have been delayed due to a FAA technical issue. No timetable yet on how long. Lovely.”

Another social media user, Autumn Johnson, shared her situation on Twitter, writing: “My flight is delayed 3hrs because of the computer outage. I’m supposed to be at work in 3hrs…”

Musician RL, or Robert Lavelle Huggar, also tweeted that he was affected by delays, writing: “Flight delayed 5 HOURS and all y’all are offering me is a $200 voucher??!! The disrespect is real. I guess it doesn’t matter that now I can’t pick my daughter up from school or that I’m missing major meetings? 2 million miler and this is the thanks I get.”

Meanwhile, other passengers were able to find light in the situation, with podcaster Mandy Matney joking: “My husband after we landed amid the FAA mess: At least we’re on the ground and it’s not a Die Hard situation.” She added: “Now hoping our next flight isn’t too delayed.”

Airports such as Philadelphia, Arkansas, and Austin are advising passengers to check in with their airline for up to date information on their journey.

What are airlines saying?

As flights slowly began to resume on Wednesday morning, airlines like American said they were “closely monitoring the situation” and encouraged customers to visit their website for the latest information. Certain carriers are offering travel waivers for the delays, and some passengers have received help related to flight and baggage delays via Twitter DMs from their airlines.

United Airlines tweeted that its offering travel waivers to impacted customers that will cover flight change fees and any differences in fare price. “Efforts are underway to minimize impacts to our customers and our operation,” the airline said.

Others, like Delta and Southwest, asked customers to stay up to date on their flight status through the app, with Delta promising that they will “provide more updates as soon as we can.”

As of 10:30 a.m., the FAA’s website indicates that Charlotte Douglas International Airport and LaGuardia are the only airports that are still experiencing ground stops. John F. Kennedy airport is also experiencing a 15-minute departure delay.