Aviation authorities are investigating why an Alaska Airlines Boeing 737 MAX 9 jet lost a door-sized section of its fuselage six minutes after takeoff from Portland, Oregon, a failure that critically depressurized the aircraft and forced an emergency landing Friday.

None of the 177 passengers and crew was seriously injured aboard Alaska Airlines Flight 1282, which was at an altitude of 16,300 feet when the fuselage unit gave way. The plane landed safely in Portland.

The National Transportation Safety Board and other agencies are investigating.

A key piece of evidence, the torn-off section known as a “door plug,” was recovered Sunday near Portland by a schoolteacher. Investigators will examine the 63-pound, 48-by-26-inch plug to determine why it separated from the plane.

Two cellphones, believed to be from the plane, were found nearby.

Authorities also are examining an auto pressurization warning light, which lit up on three previous flights − on Dec. 7, Jan. 3 and Jan. 4 − to see whether it played a part in the incident. Alaska Airlines restricted the jet from flying to Hawaii so it could land quickly if the warning light reappeared.

What happened aboard the aircraft?
Flight 1282 left Portland International Airport for Ontario, California, at about 5:07 p.m. local time Friday. Occupants heard a bang about six minutes into the flight when the fuselage section ripped away.

Passengers later described a loud, chaotic and violent scene of wind howling through the plane. The cockpit door was flung open and the wind tore the headset from the copilot. The pilot’s headset was pulled halfway off. Crew and passengers donned oxygen masks.

The FAA grounded more than 170 Boeing 737 MAX 9 jets for inspection after the incident. Alaska Airlines and United Airlines are the main operators of the MAX 9 in the U.S.

What is an aircraft door plug?
Boeing uses door plugs to cover unused emergency exits on most of its 737 MAX 9 jets. The door plug is visible from outside the plane but looks like a regular partition and window from the inside.

Boeing constructs its 737s with emergency exits behind the aircraft’s wings. That’s in accordance with federal law, which says planes must be built to allow evacuation of all passengers within 90 seconds even if half of its exits are blocked.

Airlines can choose either door plugs or emergency exits when they configure seating arrangements on the aircraft. Boeing’s 737 MAX 9 can be fitted with up to 220 seats.

Federal law requires MAX jets with more than 200 seats to have the extra emergency exits in use. Planes with fewer seats can use door plugs. The option has been an industry standard for years.

Flight 1282 has 189 seats and carried 171 passengers and six crew.

Four bolts hold the door plugs in place. The plugs have hinges at the bottom and can be unbolted and swung open about 15 degrees from the outside.