The Federal Aviation Administration on Thursday said 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 Jan. 10, the F.A.A. said that after the Portland incident, it was notified of additional issues with other Boeing 737 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 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, in Indonesia in 2018 and Ethiopia in 2019, involving the 737 Max 8 killed a total of 346 people.
The National Transportation Safety Board is investigating why the 737 Max 9 panel, also known as a door plug, flew off the Boeing jet. The safety board is trying to determine whether bolts that would have kept the panel from moving and opening were missing or were installed incorrectly. The plug is placed where an emergency exit would be if the plane had the maximum number of seats possible.
Before the announcement on Thursday, the F.A.A. had been working with Boeing on revising the company’s instructions for inspecting the grounded 737 Max 9 planes. The announcement of the revision came after reports of loose bolts from two airlines.
“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.”
Mark Lindquist, a lawyer who represented the families of victims involved in the Max 8 crashes, said that the F.A.A. was being more proactive than it had been in the past by quickly opening an investigation into the Boeing 737 Max 9. Mr. Lindquist said the F.A.A. would take a much broader look at the aircraft than 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.
