The US Federal Aviation Administration (FAA) has linked the Notice to Air Missions (NOTAM) system outage, which disrupted thousands of flights to a ‘damaged database file’.

NOTAM alerts pilots on hazards along a flight route.

The malfunction was caused by a procedural error related to a data file, which corrupted both the main system and its backup, Reuters reported citing people with knowledge of the review.

The FAA said it has no evidence of a cyber-attack.

On its website, the FAA said: “The FAA is continuing a thorough review to determine the root cause of the Notice to Air Missions (NOTAM) system outage. Our preliminary work has traced the outage to a damaged database file. At this time, there is no evidence of a cyber-attack.

“The FAA is working diligently to further pinpoint the causes of this issue and take all needed steps to prevent this kind of disruption from happening again.”

The technical glitch, which happened on Wednesday, delayed or cancelled over 11,300 flights, in what is reported to be the first national grounding of domestic traffic in nearly 20 years.

Following an overnight outage, the FAA lifted the grounding order and normal air traffic operations resumed gradually across the country.

Reuters reported that Delta Air Lines, United Airlines, American Airlines Group and Southwest Airlines have all resumed normal operations on Thursday.

However, 5,109 US flights were delayed and 163 were called off as of 7pm EST Thursday, according to flight tracking data company FlightAware.

The FAA estimates flight cancellations on Thursday to be less than 1%.