The head of the Federal Aviation Administration (FAA) is vowing that the agency will have “boots on the ground” at Boeing’s 737 MAX factory until it feels “comfortable” that its quality control system is working.

The statements from administrator Mike Whitaker come as 737-9 MAX planes remain grounded by the FAA in the wake of a Jan. 5 incident in which one of the aircraft operated by Alaska Airlines lost a passenger door plug while in flight. The FAA has since announced an “audit involving the Boeing 737-9 MAX production line and its suppliers to evaluate Boeing’s compliance with its approved quality procedures.”