The computer glitch that forced the temporary suspension last week of US domestic airline departures was caused by a contractor mistakenly deleting files, the country's aviation regulator said Thursday.

Damage to a database file had already been pinpointed by the Federal Aviation Administration (FAA) as a probable cause of the January 11 issue, which prompted flight cancellations and delays nationwide.

During the hours-long outage, pilots could not access the system known as Notice to Air Missions (NOTAM), which provides information about hazards, changes to airport facilities and information that can affect flights.

A preliminary report showed that "contract personnel unintentionally deleted files while working to correct synchronization between the live primary database and a backup database," the FAA said on Thursday.

The investigation is continuing, but the agency said it has not yet found any evidence linking the incident to malicious intent or a cyber attack.

The FAA has made the necessary repairs and taken steps to make the NOTAM "more resilient" it said.

The difficulties sparked fresh criticism on Capitol Hill and throughout Washington of the FAA, which has had no confirmed administrator since March.

The halt also came in the wake of a large-scale US aviation meltdown over the Christmas holiday, as a storm brought unseasonably cold temperatures and travel chaos to the majority of the country.