The door plug that was blown off an Alaska Airlines flight was located by a schoolteacher in Portland, authorities said late Sunday night.

National Transportation Safety Board officials announced that the teacher, identified only as “Bob,” found the missing fuselage piece in his backyard. Investigators had previously said finding that part could help them unlock clues to the midair emergency that opened a hole in the middle of the flight from Oregon to Southern California on the new Boeing 737-9 Max.

“We are really pleased that Bob found this,” NTSB Chairwoman Jennifer Homendy said. “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.

Homendy had just concluded briefing reporters on the latest details into the investigation when she called them back to the room to announce the breaking development.

There are also at least two passenger cellphones that fell out of the plane and were recovered, apparently with some of their functions intact.

One image, shows a passenger’s luggage receipt for the flight still on the screen.

Here’s why that Boeing flight had a door plug and how it is key to the investigation

The door plug fell out while the plane was at 16,000 feet and authorities said it was fortunate it happened while passengers were still required to be wearing seatbelts.

The flight from Oregon to Ontario, Calif. immediately turned around and made an emergency landing. No serious injuries were reported, though passengers reported a chaotic scene during the initial depressurization, when phones and other unsecured personal property were sucked out of the hole.

The Boeing 737-9 Max is built with an extra emergency exit that is generally only needed by foreign airlines that pack more seats and passengers into their flights. Those that don’t need it – like most U.S. domestic flights – use a door plug to cover the opening.

For example, carriers like Indonesia’s Lion Air and Corendon Dutch Airlines cram more than 200 seats into their Max 9s, so they must have those extra emergency exits. However, Alaska Airlines and United Airlines configure their 737 Max 9s to have fewer than 180 seats, so the planes don’t need the two mid-cabin exits to comply with U.S. evacuation rules.

On Alaska and United, the only two U.S. airlines using the Max 9, those side exits near the back of the plane are replaced with a permanent plug the size of an exit door.

Some passengers on Sunday said the incident makes them more wary of air travel, especially on Boeing jets.

“It’s an unacceptable thing to happen,” said Sharleen Gill, who was traveling Sunday out of Hollywood Burbank Airport. “Especially since we go through all this security to make sure we’re safe in airports. The least the airlines can do is just to make sure that the doors are working in planes and all the other parts are functioning as well.”