A missing door plug from the Boeing aircraft that had a hole blown in its fuselage in mid-flight on Friday has been found in a Portland schoolteacher’s garden.

The teacher, identified only as Bob, found the panel from the Alaska Airlines passenger jet in the city’s western Cedar Hills suburb on Sunday.

Investigators said the missing component could prove crucial to finding out why a refrigerator-sized hole had opened up shortly after the aircraft took off from Portland International Airport.

The depressurisation forced the flight, bound for Ontario, California, to make an emergency landing at Oregon 35 minutes after taking off. It had reached an altitude of about 16,000ft (4,876 metres) when it was forced to begin its descent.

All 171 passengers and six crew members on board survived, with no serious injuries.

More than 170 Boeing 737 Max 9 models in the United States have been grounded since the incident, in which phones, magazines and the shirt off a child’s back were sucked out of the plane and into the night air.

Jennifer Homendy, the chairman of the National Transportation Safety Board, confirmed on Sunday that the door plug and two missing mobile phones had been found.

Door plugs are panels in the side of a plane, and in certain configurations be used as an emergency exit if there is a large number of passengers on board.

“We’re going to go pick that up and make sure that we begin analysing it,” Ms Homendy said, but added that the cockpit’s voice recorder had been taped over.

“That is unfortunately a loss for us because that information is key, not just for our investigation, but for improving aviation safety,” she said.

The UK’s Civil Aviation Authority has demanded that all planes of the same model are inspected before entering UK airspace after the incident. The two seats next to the missing piece of fuselage were unoccupied.

“We are very, very fortunate here that this didn’t end up in something more tragic,” Ms Homendy said on Saturday.

Boeing has struggled in recent years with technical and quality control problems related to its 737 Max models. All models of the plane were grounded worldwide following two Max 8 crashes in 2018 and 2019, in which 346 people were killed.

The Federal Aviation Administration approved the planes’ return to service only after the company made changes to its flight control system.