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. The decision was supported by Boeing.

“Safety is our top priority and we deeply regret the impact this event has had on our customers and their passengers,” Boeing previously said in a statement. “We agree with and fully support the FAA’s decision to require immediate inspections of 737-9 airplanes with the same configuration as the affected airplane.”

The incident prompting the probe involved a plane that left Portland, Oregon, on Friday evening, bound for Ontario International Airport in San Bernardino County, California. Only a few minutes after takeoff, passengers said they heard “a loud bang.” A photo from a passenger on board showed a panel missing from the side of the fuselage.

The aircraft, carrying 174 passengers and six crew members, was able to turn around and land safely back at Portland International Airport.

No serious injuries were reported on the flight.

Last year, Spirit AeroSystems, the manufacturer of the door plug, was the subject of a class action lawsuit that alleged “widespread quality failures.” 

The suit, filed in New York in May by investors, didn’t specifically mention door plugs, but noted failures including defects such as “the routine presence of foreign object debris” in its products, peeling paint and missing fasteners. 

The lawsuit alleged that the company prioritized short-term financial success and production numbers over quality. Spirit AeroSystems told NBC News in a statement it “strongly disagrees” with the assertions made in the lawsuit and will “vigorously defend against the claims.”

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