A technician “erroneously” deleted information from a July inspection report about a serious track defect that caused a train to derail Aug. 6, Metro said in an investigation report released to the public on Friday.

The technician believed the problem to be a “routine anomaly,” Metro said, adding that the report “indicates a failure of [Metro’s] quality check process.” In a news release, Metro said that a supervisor and the employee responsible for deleting the defect information have resigned.

Defective rail fasteners were the root cause of the track problem known as wide gauge that caused the derailment, Metro found. The agency, formally known as the Washington Metropolitan Area Transit Authority, said the rails weren’t properly held in place, spread too far apart, and caused a wheel axle to lose its grip and derail outside Smithsonian station.

The wide gauge problem fell into a category that requires immediate attention because “the track is at risk of failure if operations continue.”

The investigation report was sent to board members earlier this week and will be the subject of a special hearing of the board on Sept. 3.

“The operations report reveals that the physical root cause of the derailment was the failure of fasteners to properly hold the rail securely,” said Metro Board Member Michael Goldman, speaking on behalf of the board in a press release. “This was compounded by a lack of systematic, immediate review of data generated during the inspection vehicle runs. The investigation is ongoing to determine what caused those fasteners to fail.  Further, the wide gauge track condition that went unrepaired was a contributory factor leading to the derailment that should never have occurred.”

A complete re-inspection of the Metro system prompted by the Aug. 6 derailment will conclude Sept. 14, Metro said in the report. The inspection will ensure no other defects were identified by equipment but deleted from technician reports.

Members of Congress whose districts are served by Metro, including Rep. John Delaney, D-Md., and Sen. Mark Warner, D-Va., heavily criticized the agency for not repairing the defect and called the agency to take action against responsible personnel when news broke earlier this month.