A passenger at Dallas-Fort Worth International Airport looks at a flight board after a system outage grounded thousands of flights throughout the country on Jan. 11.

John Moore/Getty Images

hide caption

toggle caption

John Moore/Getty Images

A passenger at Dallas-Fort Worth International Airport looks at a flight board after a system outage grounded thousands of flights throughout the country on Jan. 11.

John Moore/Getty Images

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.