F.A.A. Investigating Whether Boeing 737 Max 9 Conformed to Approved Design

The Federal Aviation Administration said on Thursday that it had opened an investigation into whether Boeing failed to ensure that its 737 Max 9 plane was safe and manufactured to match the design approved by the agency.

The F.A.A. said the investigation stemmed from the loss of a fuselage panel of a Boeing 737 Max 9 operated by Alaska Airlines shortly after it took off on Friday from Portland, Ore., leaving a hole in the side of the passenger cabin. The plane returned to Portland for an emergency landing.

“This incident should have never happened, and it cannot happen again,” the agency said.

In a letter to Boeing dated Wednesday, the F.A.A. said that after the Portland incident, it was notified of additional issues with other Max 9 planes. The letter does not detail what other issues were reported to the agency. Alaska and United Airlines, which operate most of the Max 9s in use in the United States, said on Monday that they had discovered loose hardware on the panel when conducting preliminary inspections on their planes.

The new investigation is the latest setback for Boeing, which is one of just two suppliers of large planes for most airlines. The company has struggled to regain the public’s trust after two crashes of 737 Max 8 jetliners, in Indonesia in 2018 and Ethiopia in 2019, killed a total of 346 people.

The National Transportation Safety Board is investigating why the Max 9 panel, also known as a door plug, flew off. The board is trying to determine whether bolts that would have kept the panel from moving and opening were missing or installed incorrectly. The plug is placed where an emergency exit would be if the plane had the maximum number of seats.

No one was seriously hurt in the incident, but aviation experts have said that if the panel had blown out when the plane was at a higher altitude, the consequences could have been much more severe. Passengers and flight attendants would have been walking around and may have been unable to get back to their seats to put on oxygen masks and secure their seatbelts. The Alaska Airline plane was at about 16,000 feet and still climbing when the panel tore away.

Before the announcement on Thursday, the F.A.A. had been working with Boeing on revising the company’s instructions for inspecting 171 grounded Max 9 planes. The revision was announced after Alaska and United Airlines reported the loose bolts.

“Boeing’s manufacturing practices need to comply with the high safety standards they’re…