Bob Sauer went into his backyard in suburban Portland, Ore., on Sunday night, flashlight in hand, to check if any pieces of the Alaska Airlines plane that had lost a part of its fuselage in midair had landed nearby.
A neighbor had urged Mr. Sauer to check his property in Cedar Hills, Ore., saying she had heard that a cellphone that had fallen from the plane had been found in the neighborhood.
Mr. Sauer quickly caught sight of a white metal object leaning against the branch of a cedar tree. “My heart started beating a little faster,” he said in an interview on Monday, “and I thought there’s no way.”
But it was true: Mr. Sauer, a physics teacher at the Catlin Gabel School, a nearby private school, had found the mid-cabin door plug, which had been torn from the plane mid-flight on Friday, in his yard.
He called the National Transportation Safety Board, which arrived at his house on Monday morning, interviewed him for about 30 minutes and then hauled away the critical piece of evidence from his yard, he said. The board, he said, gave him a medallion emblazoned with an eagle to thank him for his efforts.
Door plugs are used to fill emergency exits that are not needed on planes that are configured with fewer than the maximum possible number of seats. The board said in a statement on Monday that investigators were “currently examining the door plug” and planned to send it to an agency laboratory in Washington, D.C., for further examination.
The board shared photos on social media that showed the door plug in a thicket of branches and then being inspected on the ground by agents.
Mr. Sauer, 64, who has been teaching science for 40 years, said the discovery was the first thing he brought up with his astronomy class on Monday morning.
He said that the 50-foot cedar trees in his yard had acted on the same scientific principle as an airbag, disrupting the door plug’s fall — in an act known in physics as impulse.
“Impulse is what you do to change the momentum of something,” Mr. Sauer said. “You can do it with a big force over a short time, or a smaller force over a longer time.”
He said an even more relevant lesson, beyond the sheer physics of the drop, was that amazing things can happen.
“Mostly they couldn’t believe it happened in my backyard — to someone they knew,” Mr. Sauer said of his students.
Investigators are trying to determine why the plug was blown out of Alaska Airlines Flight 1282, as the plane, which had taken off from Portland, reached an altitude of about 16,000 feet, bound for Ontario, Calif.
None of the 171 passengers and six crew members aboard were seriously hurt, but they were exposed to howling winds from the hole in the fuselage as the plane returned to Portland, where it landed safely.
The episode led to the grounding of 171 Boeing 737 Max 9 planes in the United States and renewed safety concerns about the plane, which has a troubled history.
In 2018, a Boeing 737 Max 8, operated as Lion Air Flight 610, crashed off the coast of Indonesia, killing all 189 people on board. Less than five months later, in 2019, another Max 8 jet, operating as Ethiopian Airlines Flight 302, crashed shortly after leaving Ethiopia’s capital, Addis Ababa, killing all 157 people on board.
Regulators around the world grounded the Max after the second crash. The Federal Aviation Administration cleared it to fly again in late 2020 after Boeing made changes to the plane, including to a new automated system that played a role in both crashes. Boeing said in late 2019 that it had fired its chief executive, and it agreed to a $2.5 billion settlement with the Justice Department in 2021.