A corrupted database file is the suspect for a Federal Aviation Administration (FAA) outage that stopped all flights departing America for 90 minutes.
The US aviation sector was struggling to return to normal on Wednesday US time.
More than 10,000 flights have been delayed so far and over 1300 canceled, according to FlightAware, in the first national grounding of flights in about two decades.
The FAA computer issue had prevented airports from filing updated safety notices that warn pilots of potential hazards such as runway closures, poor weather and construction, bringing flights to a temporary halt.
FAA officials said a preliminary review traced the problem to a damaged database file, adding that there was no evidence of a cyber attack and that the investigation was continuing.
The same file corrupted both the main system and its backup, said people familiar with the review, who asked not to be identified.
FAA officials said they were working to “further pinpoint the causes” so the problem could be avoided in the future.
Transportation Secretary Pete Buttigieg told CNN the ground stop was the “right call” to make sure messages were getting sent correctly and there was no direct evidence of cyber attack.
The White House said President Joe Biden had confidence in Buttigieg.
Buttigieg told reporters a backup system went into effect on Tuesday but questions were raised about the system’s performance, which led to a complete reboot of the system and prompted the FAA to issue the ground stop about 7:30am EST. It was lifted just before 9 am EST.
An FAA advisory said the system that provides so-called Notices to Air Missions with safety messages for pilots and others failed about 3:30pm EST on Tuesday, which meant no new messages could be processed.
It could affect traffic through Friday, said Captain Chris Torres, vice president of the Allied Pilots Association.