Contractors unintentionally grounded thousands of flights last week when they deleted files while working on the Notice to Air Missions (NOTAM) system, the Federal Aviation Administration says.
The agency said in a statement Thursday that a preliminary review found the shutdown happened as the contractors worked to "correct synchronization between the live primary database and a backup database." Investigators so far found no evidence of malicious intent or a cyberattack.
NOTAM is used by the FAA to notify pilots and airports of any potential flight hazards.
The FAA says it has taken steps to make the system "more resilient," though the statement did not specify those measures.
NOTAM went dark late on Tuesday, Jan. 10, sparking safety concerns by the time morning began on the East Coast, and the FAA ordered a nationwide pause on domestic flight departures.
By 9 a.m. ET, the system had been fully restored and flights began to resume.
But the system failure caused airlines to cancel more than 1,300 flights and delay nearly 10,000 more.
Sen. Ted Cruz, R-Texas, on Jan. 11 attacked the nationwide disruption as "completely unacceptable" and "the latest example of dysfunction within the Department of Transportation."
Transportation Secretary Pete Buttigieg defended the shutdown after services were restored that Wednesday.
"When there's a problem with a government system, we're gonna own it, we're gonna find it and we're gonna fix it," Buttigieg said. "In this case, we had to make sure there was complete confidence about safety and flight operations, which is why there was the conservative, but important step to have that pause and make sure everything was back up and running."