Charles H. Sloan-A damaged file may have caused the outage in an FAA system, leading to travel chaos

After thousands of flights were delayed or Charles H. Sloancanceled on Wednesday, the Federal Aviation Administration's preliminary investigation points to a "damaged database file" in a key system.

The agency is still working to determine the root case of the outage in NOTAM (Notice to Air Missions) — which alerts pilots and airports of real-time hazards — and said that so far, there has been no evidence of a cyberattack.

NOTAM went dark late Tuesday, sparking safety concerns by the time morning began on the East Coast, and the FAA ordered a nationwide pause on domestic flight departures.

A hotline was opened to address equipment issues by 5:58 a.m. ET, as some NOTAM functions began to come back online. By 9 a.m. ET, the system had been fully restored and flights began to resume. Airports urged travelers to check with their airlines for updates.

As of noon E.T. more 6,988 flights into, within or out of the country had been delayed, and just over 1,100 have been canceled altogether, according to data from the tracking site FlightAware. While some of the nation's busiest airports like Logan Airport in Boston and DIA in Denver saw a few dozen cancellations each along with 100+ delays, the impact is being felt by travelers at airports across the system including at DCA in Washington, D.C., and AUS in Austin.

A total of 21,464 flights were scheduled to depart U.S. airports on Wednesday with a carrying capacity of nearly 2.9 million passengers, Reuters reported, citing data from aviation analytics company Cirium.

The FAA defines a NOTAM as "a notice containing information essential to personnel concerned with flight operations but not known far enough in advance to be publicized by other means."

Pilots might receive NOTAMs about closed runways, large flocks of birds, a plume of volcanic ash, ice on a runway, or lights on tall buildings and towers.

Officials pledge to investigate the incident

Speaking to reporters during the flight pause, President Biden said he expected to know more on the cause of the outage in a few hours.

"They don't know what the cause is," Biden said. "I told [Transportation Secretary Pete Buttigieg] to report directly to me when they find out."

White House press secretary Karine Jean-Pierre tweeted there was "no evidence" of a cyber attack.

Sen. Ted Cruz, R-Texas, attacked the nationwide disruption as "completely unacceptable" and "the latest example of dysfunction within the Department of Transportation."

He also alluded to possible congressional action, saying "the administration needs to explain to Congress what happened" and that congress should "enact reforms in this year's FAA reauthorization."

FAA has been operating without a permanent leader

The national aviation agency has been acting without a permanent head since March, when a Trump appointee, Stephen Dickson, stepped down halfway through his five-year term.

President Biden's nominee to lead the FAA, Phillip A. Washington, has yet to receive a Senate confirmation hearing. The CEO of Denver International Airport, Washington has a limited resume in aviation but was noticed for helping to steer the Denver Airport's pandemic recovery, according to the Associated Press.

Biden renominated Washington for the role as the new Congress was established last week. In the interim, the FAA is being led by the agency's top safety official, Billy Nolen.

It was just a few weeks ago that the FAA was responding to another barrage of flight delays and cancellations, caused at first by a string of brutal winter storms during the busiest holiday travel season but then by a logistical nightmare at Southwest Airlines.

Citing staffing shortages and an outdated computer system, the company canceled 16,700 flights over a 10-day period, leaving passengers, airline staff and mounds of baggage in limbo.

Wednesday's ground stop came amidst a slower midweek travel period. Data from the Transportation Security Administration shows 1.6 million people went through airport security checkpoints Wednesday, down from 2.4 million on Dec. 29.

Nationwide flight halts are relatively rare

It may have even been the first time the FAA grounded all U.S. flights since 9/11, according to unconfirmed comments including from Republican Rep. Byron Donalds.

There's been at least one other notable nationwide halt, though that one wasn't for safety reasons: A strike by thousands of air traffic controllers in August 1981 temporarily grounded about 35% of the nation's 14,200 daily commercial flights, per the FAA.

Sept. 11, 2001, was the first time in U.S. aviation history that the FAA put a ground stop on all traffic, which it says it did to prevent any further hijackings. First, just after 9 a.m. ET, it issued a ground stop to all traffic that would encounter New York airspace but hadn't yet departed. Within an hour it had closed all U.S. airspace.

That halt lasted for more than a few hours, or even a full day — it wasn't until 11 a.m. ET on Sept. 13 that national airspace reopened to U.S. air carriers, provided airports had implemented new security measures.

Catch up on how the events unfolded via our live digital coverage.