WASHINGTON — The missing part of the Alaska Airlines plane that detached while the plane was in the air on Friday has been found, investigators say.

The authorities had been searching for the door plug in Portland — the city the Boeing 737 Max 9 had just departed.

A teacher called Bob found the missing section in his yard, the chair of the organization leading the investigation said on Monday.

Some 171 Boeing 737 Max 9 planes remain grounded as safety checks continue.

Others were returned to service on Sunday, with the airline saying an inspection found "no concerning findings".

It apologized to those affected by the grounding of the jets.

The plane affected on Friday made an emergency landing in Portland after losing a section of fuselage mid-flight.

The flight to Ontario, California, had reached 16,000ft (4,876m) when it began its emergency descent, according to flight tracking data.

One passenger, Diego Murillo, said the gap was "as wide as a refrigerator".

It landed safely back in Portland, with no harm to its 177 passengers and crew.

Earlier, the chair of the organisation leading the investigation into the incident revealed there had been previous warning alerts on the jet.

Jennifer Homendy, of the US National Transportation Safety Board (NTSB), said pilots had reported pressurisation warning lights on three earlier flights in the days before Friday's incident.

It is not yet clear if there is a link.

Ms Homendy described the event as "very chaotic", adding that no information was available to read from the cockpit voice recorder because it was not retrieved before the two-hour mark. — BBC