The Federal Aviation Administration announced an investigation into Boeing after an Alaska Airlines flight was forced to make an emergency landing when a door plug fell off the fuselage midair.

“This incident should have never happened and it cannot happen again,” the FAA said in a statement Thursday.

The administration said it notified Boeing of its investigation, which will look into whether Boeing “failed to ensure completed products conformed to its approved design and were in a condition for safe operation in compliance with FAA regulations.”

“This investigation is a result of an incident on a Boeing Model 737-9 MAX where it lost a ‘plug’ type passenger door and additional discrepancies,” the statement said. “Boeing’s manufacturing practices need to comply with the high safety standards they’re legally accountable to meet.”

The FAA had previously ordered the temporary grounding of some Boeing 737 Max 9 aircraft for inspections, affecting about 171 planes worldwide.

This is a developing story. Please check back for updates.