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 legally accountable to meet,” the F.A.A. said in the statement announcing the investigation.
Dave Calhoun, Boeing’s chief executive, on Tuesday promised transparency in the company’s response to the incident. He also said the company was “acknowledging our mistake” without explaining what he was referring to. Boeing has declined to elaborate on that remark.
“We will cooperate fully and transparently with the F.A.A. and the N.T.S.B. on their investigations,” Boeing said in a statement.
United has 79 of the planes and Alaska has 65, but Alaska has had the bigger share of cancellations from the grounding because the Max 9 makes up 20 percent of its fleet.
Arjun Garg, a former chief counsel and acting deputy administrator of the F.A.A., said that by notifying Boeing of its investigation, the agency had set in motion a process that could result in an enforcement action against the company. In other such cases, the F.A.A. has imposed fines and reached agreements requiring companies to make changes to fix problems the regulators have identified.
“Ultimately, the F.A.A. is interested in people being safe, not in collecting penalties or anything like that,” said Mr. Garg, now a partner at the law firm Hogan Lovells in Washington. “They just want to make the system safer.”
Some lawmakers are also asking whether the F.A.A. made mistakes.
Senator Maria Cantwell of Washington State, the Democrat who leads the Commerce Committee, asked the agency on Thursday to provide the committee with an accounting of its management of manufacturers’ compliance with quality control standards.
“In short, it appears that F.A.A.’s oversight processes have not been effective in ensuring that Boeing produces airplanes that are in condition for safe operation, as required by law and by F.A.A. regulations,” Ms. Cantwell said in the letter to the agency’s administrator, Mike Whitaker.
The F.A.A. declined to comment on the senator’s letter, saying it would respond directly to Ms. Cantwell.
A lawyer who represented the families of victims involved in the Max 8 crashes, Mark Lindquist, said the F.A.A. was being more proactive than it had been in the past by quickly opening an investigation. He said the F.A.A. would take a much broader look at the Max 9 than the N.T.S.B., which aims to establish the cause of accidents and makes recommendations for how they can be prevented.
“The tone of this announcement indicates the F.A.A. believes there was the potential for loss of life and the seriousness of the Boeing quality control issues,” Mr. Lindquist said.
The F.A.A. had to move quickly because it could not afford to have travelers worrying about the safety of Boeing planes, said Robert Mann, a former airline executive who is now an aviation industry consultant.
Mr. Mann said the F.A.A. would probably take a hands-on approach to inspecting the Max 9 planes as it did with the Max 8. He noted that Steve Dickson, who was the agency’s administrator at the time and a former airline pilot, flew the Max 8 before the F.A.A. allowed commercial flights on the jets in late 2020 after they were grounded for nearly two years.
“This is a recognition of a very longstanding problem and is a very public rebuke,” Mr. Mann said.
The F.A.A.’s investigation provides an opportunity for Boeing and the agency to make sure they have determined whether issues with the Max 9 planes are isolated or systematic, said Billy Nolen, a former acting administrator of the agency. “That’s something they will know by the time they get through some of these 171 aircraft,” he said.
Mr. Nolen said the F.A.A. is responsible for making sure every component of an aircraft meets the agency’s standards. The fact that airlines have found loose bolts on other Max 9 jets provides ample reason for the agency to open an investigation.