A corrupted computer file led to the breakdown of an air-safety system that prompted flights to be grounded across the U.S., according to people familiar with the preliminary findings.

The glitch that affected the Notice to Air Missions, or Notam, system January 11 also caused a failure in a related backup system, said the people, who asked not to be identified discussing the ongoing investigation.

They cautioned that the information was not final.

The computer system that shares the notices to pilots, airlines and other users began developing problems late on the night of January 10, and had to be completely taken down in the early hours of January 11. The Federal

Aviation Administration temporarily halted domestic departures, leading to thousands of flight delays.
Technology workers tried to activate a backup system and it initially seemed to function, but the same or a similar corrupted file caused problems there as well, said one of the people.

A halt to all flights across the country is extremely rare and has only been done a handful of times, such as after the September 11, 2001, terrorist attacks.

Authorities haven’t said what caused the system breakdown, but statements from the White House and Secretary of Transportation Pete Buttigieg said there wasn’t evidence of a cyberattack.