Investigators were searching Sunday for the piece of fuselage that blew off a Boeing airliner over Oregon on Friday, hoping to gain physical evidence of what went wrong.

The gaping hole in the side of the Alaska Airlines jet opened up where aircraft maker Boeing fits a “plug” to cover an emergency exit that the airline does not use.

The plugs are on most Boeing 737 Max 9 jets. The Federal Aviation Administration has temporarily grounded those planes until they undergo inspections of the area around the door plug.

WHY THE PLUG IS THERE

Some larger Boeing 737s have emergency exits on fuselages behind the wings to meet a federal requirement that planes be designed so passengers can evacuate within 90 seconds even if half the exits are blocked.

The more passenger seats there are on a plane, the more exits are required.

Some carriers, including Indonesia’s Lion Air and Corendon Dutch Airlines, cram more than 200 seats into their Max 9s, so they must have 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.

ARE THEY ONLY ON MAX 9s?

No. Boeing also makes bigger versions of its 737-900 — a predecessor to the Max — and the Max 8 with space for extra exits in the back. Buyers of those planes also may opt to have either exit doors or plugs installed.

WHO INSTALLS THE PLUGS?

A spokesman for Spirit AeroSystems — which is unrelated to Spirit Airlines — confirmed to The New York Times that the company installed door plugs on Max 9s, including the plug on the Alaska Airlines plane involved in Friday’s incident. The Seattle Times reported that door plugs are assembled into 737 fuselages at Spirit’s factory in Wichita, Kansas.

The Spirit AeroSystems spokesman did not respond to questions from The Associated Press. Boeing declined to comment on the issue.

THE BOEING SUPPLIER

Spirit is Boeing’s largest supplier for commercial planes and builds fuselages and other parts for Boeing Max jets. The company has been at the center of several recent problems with manufacturing quality on both the Max and a larger plane, the Boeing 787 Dreamliner. Last year, Boeing and Spirit AeroSystems discovered improperly drilled fastener holes in a bulkhead that keeps 737 Max jets pressurized at cruising altitude.

THE INVESTIGATION

Officials with the National Transportation Safety Board, led by the board’s chair, Jennifer Homendy, arrived in Portland, Oregon, on Saturday to begin an investigation that is likely to last a year or longer. Homendy declined to discuss possible causes when she briefed reporters on Saturday night.

The NTSB team includes a metallurgist, and Homendy said investigators will look at the exit-door plug if they can find it, as well as its hinges and other parts.

Examining the damage to the door will be crucial to the investigation, according to independent experts.

“The good thing about metal is that metal paints a picture, metal tells a story,” said Anthony Brickhouse, who teaches accident investigation at Embry-Riddle Aeronautical University in Daytona Beach, Florida. “I’m pretty confident they will find the piece that came off, and they will be able to speak to scientifically what happened to cause this failure.”

Brickhouse said the exit doors, whether plugged or not, are not necessarily a weak point in the fuselage. He had never heard of an exit door plug falling off a plane before Alaska Airlines flight 1282.

WERE THERE WARNINGS?

Aerospace analysts for the investment bank Jefferies wrote that the plane involved in Friday’s incident experienced pressurization issues on two earlier flights. The NTSB has not commented on the plane’s history, but Homendy said investigators would examine maintenance records even on such a new plane.

OTHER FUSELAGE BLOWOUTS

There have been rare instances of holes opening in the fuselages of airliners. In most cases, they have been the result of metal fatigue in the plane’s aluminum skin.

In the most horrific case, a flight attendant for Aloha Airlines was blown out of the cabin of a Boeing 737 over the Pacific Ocean in 1988 after an 18-foot-long chunk of the roof peeled away. Her body was never found. The tragedy led to tougher rules for airlines to inspect and repair microscopic fuselage cracks before they tear open in flight.

In 2009, a hole opened in the roof of a Southwest Boeing 737 flying 35,000 feet over West Virginia. And in 2011, a 5-foot-long gash unfurled in another Southwest Boeing 737, forcing pilots to make an emergency landing at a military base in Arizona. No one was injured in either of those cases, both of which were blamed on metal fatigue.