U.S. air traffic was mostly back to normal Thursday, a day after a computer system sent safety alerts to pilots that grounded coast-to-coast flights.
About 150 flights were canceled and more than 3,700 were delayed on the East Coast by midday, a significant decrease from Wednesday, when more than 1,300 flights were canceled and 11,000 were delayed.
The focus is on the federal agency whose technology glitch appears to have started hours before it disrupted more than a million commuters.
The Federal Aviation Administration said a corrupted database file appears to have caused the security alert system to fail. Transport Secretary Pete Buttigieg has promised a full investigation to avoid another major blackout.
Buttigieg said in a statement to reporters, "Our immediate concern is the development of understanding how this happened, why the redundancy and mitigation built into the system could not prevent the rate of decline we saw."
Buttigieg said there was no indication the crash was the result of a cyber attack, but officials were not ruling it out until they knew more.
The FAA said late Thursday that a pre-investigation analysis showed the crash occurred after "data file corruption by personnel who did not follow procedures."
The mass outage is the latest black eye for the agency, which has accused airlines of causing huge inconvenience to passengers. Critics, including major airlines and travel companies, say agent technology is financially underfunded.
"Certainly investment is needed," American Airlines CEO Robert Isom told CNBC. It will cost billions of dollars, and it won't happen overnight.
United Airlines CEO Scott Kirby has criticized the FAA on several issues, including the appointment of air traffic controllers. The agency says it makes a "valiant effort" and that it usually works well, but can get overwhelmed at peak travel times.
"They need more investment in technology," Kirby said at a US Chamber of Commerce event in September. "You said so."
Washington state Rep. Rick Larson said the shutdown, the top Democrat on the House Aviation Subcommittee, shows how weak the FAA's technology is and that the agency needs to make major reforms.
"It's one thing to do something with old programs," Larson said in an interview. "Investing in new software platforms to prevent this from happening again is another matter."
Mike McCormick, a former FAA air safety officer who retired in 2017 after 35 years at the agency, was more confident in FAA technology. He said the agency has upgraded its IT systems over the past 15 years, 95 percent of which have been upgraded to the next generation satellite system for navigation, flight tracking and communications.
"The software, the hardware and the latest upgrades have been completed in the last three years, so they're currently working on the next generation and upgrading the system," said McCormick, who teaches air traffic management at Embry. Enigma Aviation University. .
The system that generates the NOTAM, or air mission notification, was also updated, but the failure occurred when an engineer was working on the main system and the database was corrupted, McCormick said, citing conversations with FAA personnel.
According to McCormick, when they switched to a backup system, the database was also damaged. Then the system should be rebooted.
"It could still be wrong," McCormick said. "You can still have human errors, you can still have process errors, you can still have technology errors."
Michael Huerta, FAA administrator from 2013 to 2018, said systems must be updated regularly to keep pace with changing technology. There is no age-related risk of a crash in the FAA system, specifically a system that tracks and communicates with aircraft.
"Indeed, the public should be satisfied that the air traffic control system is reliable," he said.
But the NOTAM system has been around for decades by the time vendors no longer support it or the platform it runs on has evolved.
"This is not a one-time event," he said. "It won't be many years before you have to improve it."
The shutdown comes at an inopportune time for the FAA and Buttigieg.
The FAA is trying to rebuild its reputation after the Boeing 737 Max was widely criticized for failing to fully understand its flight control system, which played a key role in two crashes that killed 346 people. The agency took a more hands-on approach, reviewing and ultimately improving the changes Boeing made to bring the plane back.
Breaking up the agency, which is overseen by the Department of Transportation, could also dampen Buttigieg's morale of punishing airlines when they cancel or delay flights. He has been suing airlines since last summer, most recently over layoffs at Southwest Airlines.
Wednesday's controversy showed how much U.S. air travel depends on the computerized system that generates NOTAMs.
Before takeoff, pilots and air traffic controllers should review messages containing information about weather, runway closures, or other temporary conditions affecting flight. The system used to work over the phone, but that was many years ago.
When Buttigieg's system went down on Tuesday night, the backup system came online. The FAA tried to restart the main system Wednesday morning but was unsuccessful, prompting the FAA to take the rare step of grounding the planes.