US air safety officials say that the glitch that led to travel chaos at airports last week was actually caused by a contractor deleting files on a crucial computer server used by pilots.
The Federal Aviation Administration (FAA) said the worker “unintentionally deleted files” on the Notice to Air Missions (Notam) database.
The system alerts pilots to potential hazards on flight routes. They are required to check it before flights.
Lawmakers vowed to look into the issue.
More than 11,000 flights were delayed and at least 1,300 were cancelled on 11 January after the Notam system went offline a day earlier.
The technical issues marked the first time since the attacks on 11 September 2001 that flights across the US were grounded.
The FAA said that their contract employee, who was not identified, deleted the files while working to synchronise the primary and backup Notam databases.
“The agency has so far found no evidence of a cyber-attack or malicious intent,” the regulator said in its statement on Thursday.
The FAA added that they were continuing to investigate the error. The system has been fixed, and the FAA “has taken steps to make the Notam system more resilient,” the statement said.
The FAA had previously attributed the outage to a “damaged database file”.
Last week, a group of Washington DC lawmakers wrote to the FAA to say that the outage was “completely unacceptable” and demanding to know how it would be avoided in the future.
FAA acting Administrator Billy Nolen plans to hold a virtual briefing for lawmakers on Friday to discuss their concerns.