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Federal investigators said late Monday that it was possible that the bolts that were supposed to keep a fuselage panel in place were never installed before the panel blew off an Alaska Airlines Boeing 737 Max 9 in a near-disastrous accident on Friday night.

That is one of the theories that the National Transportation Safety Board is pursing as it investigates the blowout, the board’s chairwoman, Jennifer Homendy, said at a news conference in Portland, Ore. Her remarks came hours after United Airlines said it had found loose bolts on similar panels on some of its Max 9 jets while preparing them for inspection after the midair emergency, and Alaska Airlines said it had also found “loose hardware” on Max 9s.

The panel that came off the plane, called a door plug, is placed where an emergency exit door would be if a jet had more seats. Ms. Homendy said on Monday that four bolts, known as stop bolts, should have prevented the door plug from moving upward and coming off the plane.

But the bolts were not on the door plug when investigators recovered it, and they are trying to determine whether they were there to begin with. “We don’t know if they were there or if, again, they came out during the violent explosive decompression event,” Ms. Homendy said.

The door plug came off the plane, Alaska Airlines Flight 1282, about 10 minutes after it took off from Portland International Airport, subjecting passengers to howling wind and forcing the pilots to quickly return to the airport. No serious injuries were reported. The door plug, phones, toys and other personal items all streamed out of the hole in the side of the plane and fell across Portland.

Airlines have canceled hundreds of flights as they prepare to inspect nearly 200 Max 9 aircraft, which will be grounded until regulators and company officials decide they are safe. Some passengers’ travel plans could be disrupted for days. Alaska Airlines used 65 of the planes, about 20 percent of its fleet, and United used 79, more than any other airline and about 8 percent of its fleet, according to Cirium, an aviation data provider.

United Airlines said on Monday that it had found loose bolts in door plugs in some of its Max 9 planes as it took out seats and sidewall liners for inspections this weekend.

The door plug that came off the plane was initially installed by Spirit AeroSystems, which makes the body for the 737 Max and other aircraft. Investigators said they were looking into whether work had been carried out on or near the door since the plane entered service in November.

Ms. Homendy said on Sunday that Alaska Airlines had been warned three times before the Friday flight about problems with cabin pressure on the plane. Those warnings were significant enough that the airline had decided the plane, a Max 9, could no longer be used on flights to Hawaii.

Investigators may look into whether the installation of wireless internet equipment on the plane by a contractor, AAR, between Nov. 27 and Dec. 7 played any part in the pressurization problems, which emerged after that work was complete. In a statement, AAR said on Monday that it “did not perform any work on or near any midcabin exit door plug of that specific aircraft.”

The accident Friday could have been far more catastrophic, especially if the plane had been at a higher altitude, experts said. Ms. Homendy said on Sunday night that the passengers had included three babies and four unaccompanied children between the ages of 5 and 17.

Ms. Homendy said in a brief interview on Monday that her team was reviewing the plane’s flight data recorder to try to determine if the pressurization warning light could be linked to the door plug. The plane has several backup systems in case one of the pressurization systems fails.

“There may have been something wrong with either the light or that one other unit, but there’s redundancies in the system,” Ms. Homendy said.

Kathleen Bangs, an aviation expert and a former airline pilot, said she believed that the investigation would reveal a failure of the door plug because of the condition of the plane. Typically, explosive decompression incidents happen on older planes that have metal corrosion and fatigue, Ms. Bangs said. In this case, she said, the plane was almost new, which indicates that there was most likely an issue with the door plug.

Anthony Brickhouse, a professor of aerospace safety at Embry-Riddle Aeronautical University, said a blowout at a cruising altitude of more than 30,000 feet could have been disastrous. “We could have been looking at a situation where more of the structure could have come off and would have been looking at a situation where passengers who weren’t strapped in properly would have been blown out because the forces would have been so tremendous,” he said.

Pressurization starts to affect most commercial planes around 8,000 feet, said Mr. Brickhouse, who previously investigated aviation accidents for the safety board. Not properly controlling the air entering and leaving the cabin can lead to altitude sickness, or hypoxia, among passengers and the crew.

Hypoxia, a condition that develops when the brain is deprived of oxygen, can happen on planes without appropriate pressurization when they begin flying above 10,000 feet or suffer rapid decompression, the F.A.A. says. This is why flight attendants tell passengers to use drop-down masks in the event of rapid decompression, Mr. Brickhouse said.

In a statement, the F.A.A. said the required inspections would concentrate on the plugs, door components and fasteners.

“Our teams have been working diligently — with thorough F.A.A. review — to provide comprehensive, technical instructions to operators for the required inspections,” Stan Deal, the chief executive of Boeing’s commercial plane unit, and Mike Delaney, the chief aerospace safety officer, said in a message to employees of that unit on Monday.

Other airlines with Max 9 planes are outside the United States, such as Copa Airlines of Panama, Turkish Airlines and Icelandair. The European Union’s aviation safety agency announced on Monday that the Max 9 jets operating in Europe were not grounded because they had a different configuration.

The F.A.A. previously said it would take four to eight hours to inspect each plane. Inspecting the nearly 200 Max 9 planes in the United States, according to the aviation agency, could take a few days.

Aviation regulators and Boeing said the inspections were unique to the Max 9. The Max 9, along with the more popular Max 8, was grounded for nearly two years after two crashes of the Max 8 in 2018 and 2019 killed 346 people.

In a statement, Alaska Airlines said it could not answer many outstanding questions about the plane and what had led to the blowout without approval from the safety board. The airline said it had asked the N.T.S.B. to share more information and would do so if allowed. In such investigations, parties are typically restricted in what they can share publicly.

Boeing’s chief executive, Dave Calhoun, planned to host a companywide safety meeting on Tuesday to discuss the company’s response to the episode and reaffirm its commitment to safety. Boeing is still working to secure approval of the smaller Max 7 and larger Max 10.

Boeing shares closed down about 8 percent on Monday, and shares of Spirit AeroSystems closed down 11 percent.

J. Edward Moreno contributed reporting.

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