Following a preliminary review, the US Federal Aviation Administration has now revealed contractors "unintentionally deleted files" while trying to sync the US Notice to Air Missions (Notam) system – grounding all US flights for several hours in the process on Wednesday (11 January).

Data from flight schedule analyst Cirium suggested around 21,500 flights had been due to depart US airports on the day of the outage.

A fix has been rolled out, along with additional measures to make the system more resilient. The FAA had previously put the issue down to a "damaged database file". It has also ruled out foul play for the moment.

The Notam system provides pilots essential hazard and security updates affecting routes, and must be checked prior to take-off.

An FAA statement read: "A preliminary FAA review of last week’s outage of the Notice to Air Missions system determined contract personnel unintentionally deleted files while working to correct synchronisation between the live primary database and a backup database.

"The agency has so far found no evidence of a cyber attack or malicious intent. The FAA continues to investigate the circumstances surrounding the outage."

It added: "The FAA made the necessary repairs to the system and has taken steps to make the Notam system more resilient. The agency is acting quickly to adopt any other lessons learned in our efforts to ensure the continuing robustness of the nation’s air traffic control system."