It was discovered in a suburban backyard in Portland, Oregon by a schoolteacher named Bob.

The chair of the National Transportation Safety Board, Jennifer Homendy:

"I know it's unusual to come back a second time but I wanted to make sure that you had all this information in real-time. I'm excited to announce that we've found the door plug. Thank you Bob."

Homendy said the aircraft part was a "key missing component" to determine why the accident occurred.

On Friday, a plug door tore off the left side of the Boeing 737 MAX 9 following takeoff from Portland.

It immediately depressurized the aircraft, and forced the pilots to turn back and land safely with all 177 passengers and six crew on board.

As the investigation goes on, the Federal Aviation Administration ordered the temporary grounding of 171 Boeing jets installed with the same panel.

It weighs about 60 pounds and covers an optional exit door mainly used by low-cost airlines.

Alaska Airlines on Sunday canceled 170 flights affecting nearly 25,000 customers.

The accident has put Boeing back under scrutiny as it awaits certification of its smaller MAX 7 and the larger MAX 10 aircraft.

In 2019, global authorities subjected all planes of the Boeing 737 MAX model to a wider grounding after deadly crashes in Ethiopia and Indonesia.