Investigators find blown out door plug from Alaska Airlines plane, Photo: STEPHEN BRASHEAR / GETTY IMAGES NORTH AMERICA / GETTY IMAGES VIA AFP

The National Transportation Safety Board (NTSB) chief said a teacher, named only as "Bob," found the panel in his backyard in the city of Portland, in the US state of Oregon.

"I am excited to announce we have found the door plug," NTSB chair Jennifer Homendy said at a press briefing.

"We are really pleased that Bob found this," she added.

"He took a picture. I can just see the outside of the door plug from the pictures, the white portions. We can't see anything else but we're going to go pick that up and make sure that we begin analyzing it."

Airlines and safety bodies around the world grounded some versions of the Boeing 737 MAX 9 jets pending inspections, with dozens of flights canceled after the incident.

The Federal Aviation Administration "is requiring immediate inspections of certain Boeing 737 MAX 9 planes before they can return to flight," the agency said on X.

It added that around 171 aircraft worldwide would be affected, with each inspection taking four to eight hours.

US-based Alaska and United Airlines fly the largest number of MAX 9 planes of any carrier and said on Sunday they had grounded their aircraft for inspection.

Other airlines with smaller MAX 9 fleets, including Turkish Airlines, said they did the same.

Boeing has so far delivered about 218 of the 737 MAX planes worldwide, the company told AFP.

Alaska Flight 1282 had departed from Portland International Airport and was still gaining altitude when the cabin crew reported a "pressurization issue," according to the FAA.

The plane quickly returned to Portland and there were no major injuries.

Images posted on social media showed a gaping hole where the side panel had blown out, with emergency oxygen masks hanging from the ceiling.

The plane manufacturer late Sunday said its chief executive Dave Calhoun has set an all-employee safety meeting for Tuesday at the company's factory in Washington state, and canceled an overlapping leadership summit.

"In light of the Alaska Airlines Flight 1282 in-flight event, we are canceling the Summit to focus on our support to Alaska Airlines and the ongoing...(NTSB)investigation, and any of our airline customers experiencing impact to their fleets," Calhoun said in a Boeing statement.

- 'Very, very fortunate' -

Passenger Kyle Rinker told CNN the problem occurred soon after takeoff.

"It was really abrupt. Just got to altitude, and the window/wall just popped off," he told the broadcaster.

The NTSB said no one occupied the two seats nearest the panel, but the Oregonian newspaper quoted passengers as saying a young boy seated in the row had his shirt ripped off by the sudden decompression, injuring him slightly.

According to Aviation Week magazine, airlines that choose MAX models with smaller seating configurations can have the door sealed up, making it look like a typical window from the inside.

The NTSB dispatched a team to Portland to examine the Alaska Airlines craft.

The board's chair, Jennifer Homendy, said it was "very, very fortunate" that the incident had not ended in tragedy.

"We have the safest aviation system in the world. It is incredibly safe," she said. "But we have to maintain that standard."

Homendy revealed that the panel had fallen off over the Portland suburb of Cedar Hills, and urged residents to come forward if they found it.

The plane, which had been heading to Ontario, California, was certified airworthy in October and was newly delivered to Alaska Airlines, according to the FAA registry website.

"Safety is our top priority and we deeply regret the impact this event has had on our customers and their passengers," Boeing said in a statement.

- MAX struggles -

United said 33 of its 46 grounded MAX 9 planes had now been inspected. There were an estimated 60 flight cancellations on Saturday.

Aeromexico said it was grounding all of its 737 MAX 9 planes while inspections were carried out, while Panamanian carrier Copa Airlines said it was grounding 21 of its MAX 9s.

Icelandair said none of its MAX 9's featured the plane configuration specified in the FAA grounding order.

The European Union's Aviation Safety Agency said on Sunday it would follow the FAA's directive, but that it does not believe any EU airlines currently operate the 737 MAX 9.

Boeing has struggled in recent years with technical and quality control issues related to its 737 MAX models.

In December, the US aviation giant told airlines that MAX aircraft should be inspected to check for loose hardware on plane rudder control systems after an international operator discovered a bolt with a missing nut while performing routine maintenance.

Boeing's 737 MAX planes were grounded worldwide following two MAX 8 crashes in 2018 and 2019 that killed 346 people in total.

The FAA approved the planes' return to service only after the company made changes to its flight control system.