As air travel in the US returned to normal after a major disruption in airline operations, the Federal Aviation Administration (FAA) on Thursday said that the computer outage was caused by a procedural error related to a data file.

The FAA said its preliminary analysis "determined that a data file was damaged by personnel who failed to follow procedures. The system is functioning properly."

The massive airway disruption that occurred on Wednesday led to the first national grounding of domestic traffic since the September 11, 2001, terrorist attacks.

More than 11,300 flights were either delayed or cancelled as airports were unable to file updated safety notices that warn pilots of potential hazards, due to the computer breakdown.

Seeking a 'technical understanding'

Ruling out a cyberattack, the FAA had earlier said that the fault occurred because of a damaged database file in the system that provides pilot safety notices known as Notices to Air Missions (NOTAMs).

Transportation Secretary Pete Buttigieg pledged a thorough investigation. "Our immediate focus is technical understanding exactly how this happened, why the redundancies and the backups that were built into the system were not able to prevent the level of disruption that we saw," said Buttigieg.