• The FAA says a contractor unintentionally deleted files before the outage of alert system.
• Around 10,000 flights were delayed because of the pilot-alerting system outage.

The Federal Aviation Administration said Thursday that a contractor unintentionally deleted files before an outage of a pilot-alert system that delayed thousands of flights last week.

"A preliminary FAA review of last week's outage of the Notice to Air Missions (NOTAM) system determined that contract personnel unintentionally deleted files while working to correct synchronization between the live primary database and a backup database," the FAA said. Spokespeople for the agency didn't provide further detail.

Those notices give pilots safety information such as runway closures.

The FAA reiterated that it hasn't found evidence of a cyberattack or "malicious intent" and that it is still investigating what occurred.

The agency said it updated lawmakers on its investigation on Thursday. Lawmakers from both parties demanded answers about technology vulnerabilities in the U.S. aviation system.

Airline executives complained about inadequate funding and staffing for the FAA.

"I lay this on the fact that we are not giving them the resources, the funding, the staffing, the tools, the technology they need," Delta Air Lines CEO Ed Bastian said on "Squawk Box" on Friday. "Hopefully this will be the call to our political leaders in Washington that we need to do better."

United Airlines CEO Scott Kirby said on the company's earnings call on Wednesday the outage and resulting travel chaos "ought to be a wake-up call for all of us in aviation, something many of us in aviation have been saying for a long time...the FAA needs more resources."