U.S. air travel returned mostly to normal Thursday, a day after a computer system that sends safety information to pilots broke down and grounded traffic from coast to coast.

By midafternoon on the East Coast, about 150 flights had been canceled and more than 3,700 delayed — much lower figures than on Wednesday, when more than 1,300 flights were scrubbed and 11,000 delayed.

Attention turned to the federal agency where the technology failure apparently started hours before it inconvenienced more than 1 million travelers.

The Federal Aviation Administration said a damaged database file appeared to have caused the outage in the safety-alert system. Transportation Secretary Pete Buttigieg promised a thorough examination to avoid another major failure.

“Our immediate focus is technical — understanding exactly how this happened, why the redundancies and the backups that were build into the system were not able to prevent the level of disruption that we saw,” Buttigieg told reporters.

Buttigieg said there was no indication that the outage was caused by a cyberattack but that officials would not rule that out until they know more.

Read more: Flight disruptions cascade across US after computer outage

The FAA said late Thursday that a preliminary analysis showed the breakdown came after “a data file was damaged by personnel who failed to follow procedures.”

The massive disruption was the latest black eye for the agency, which has traded blame with airlines over who has inconvenienced passengers more. Critics, including airline and tourism leaders, say agency technology is underfunded.

“Investment is going to be required, no doubt,” American Airlines CEO Robert Isom told CNBC. “It’s going to be billions of dollars, and it’s not something that is done overnight.”

United Airlines CEO Scott Kirby has been critical of the FAA on a variety of issues, including staffing of air traffic controllers. He says the agency makes “a heroic effort” and does well most of the time but can be overwhelmed during busy travel times.

“They need more investment for technology,” Kirby said at a U.S. Chamber of Commerce event in September. “They have been saying it."

Rep. Rick Larsen of Washington state, the top Democrat on a House aviation subcommittee, said the outage shows the weakness of the FAA's technology and that the agency needs to make significant improvements.

“It's one thing to get things up and going on the old software,” Larsen said in an interview. “It's another thing to invest in the new software platforms that are necessary to ensure this doesn't happen again.”

Mike McCormick, a former FAA manager of airspace security who retired in 2017 after about 35 years at the agency, was more confident in FAA technology. He said the agency modernized computer systems over the past 15 years and is 95% up to date, having upgraded to next-generation satellite-based systems for navigation, flight tracking and communication.

“Software, hardware, the final upgrades, were completed in the last three years, so now they’re actually working on the next generation beyond that and the enhancements to the systems,” said McCormick, who now teaches air-traffic management at Embry-Riddle Aeronautical University.

The system that generates NOTAMs — or Notice to Air Missions — also was upgraded, but the outage happened as an engineer was working on the main system and the database somehow became corrupted, McCormick said, citing conversations with people at the FAA.

When they switched to the backup system, its database also was corrupted, McCormick said. The system then had to be rebooted.

Read more: Number of flights canceled by Southwest Airlines is growing

“Things can still go wrong,” McCormick said. “You can still have human error, you can still have procedural errors, you can still have technological errors.”

Michael Huerta, FAA administrator from 2013 to 2018, said the systems need to be updated constantly to keep up with technology. Nothing in the FAA system is so old that it's in danger of failing, he said, especially the system that tracks and communicates with planes.

“The public should definitely be confident that the air traffic control system is safe,” he said.

But the NOTAM system is about a decade old when systems reach the point where vendors don't support it or the platform that it runs on has been upgraded.

“It's not a one-and-done type of event,” he said. “It's not very many years that go by before you have to upgrade it.”

The outage came at a bad time for both the FAA and Buttigieg.

The FAA is trying to repair its reputation after being widely criticized for the way it approved the Boeing 737 Max without fully understanding a flight-control system that malfunctioned and played a key role in two crashes that killed 346 people. The agency took a more hands-on approach when considering — and eventually improving — changes that Boeing made to get the plane back in the air.

The meltdown at an agency overseen by the Transportation Department could also undercut Buttigieg’s moral authority to chastise airlines when they cancel or delay flights. He has gone after the airlines since last summer, most recently over disruptions at Southwest Airlines.

Wednesday's breakdown showed how much American air travel depends on the computer system that generates NOTAMs.

Before a plane takes off, pilots and airline dispatchers must review the notices, which include details about weather, runway closures or other temporary factors that could affect the flight. The system was once telephone-based but moved online years ago.

Buttigieg said when the system broke down Tuesday night, a backup system went into effect. The FAA tried a complete reboot of the main system Wednesday morning, but that failed, leading the FAA to take the rare step of preventing planes from taking off.