A catastrophic computer system failure that led to more than 11,000 flights across the U.S. being disrupted last week was triggered after contract personnel “unintentionally deleted files,” the Federal Aviation Administration said Thursday. The government agency launched an investigation into how its vital NOTAM messaging system glitched on Jan. 11, causing the first nationwide grounding of flight departures since the Sept. 11, 2001, attacks. The FAA said the problem began when contract staff were working to “correct synchronization between the live primary database and a backup database.” The agency added that it “has so far found no evidence of a cyber-attack or malicious intent.”