Investigators are assessing a piece of fuselage that came off an Alaska Airlines Boeing 737 MAX 9 jet shortly after it took off from Portland, Oregon.

The missing panel is a plug that is installed on some 737 MAX 9s in place of an additional emergency exit.

As a result, regulators have grounded 171 planes so that airlines can conduct inspections of those crafts.

Some airlines have already discovered that the bolts used to secure the panel were loosened. Here is an explanation of that panel.

Alaska Airlines CEO ‘anxious’ for Boeing 737 MAX 10 deliveries …

What we know

As Alaska Air Flight 1282 reached just over 16,000 feet, the panel tore off from the side of the jet, leaving a rectangular hole the size of a refrigerator in the aircraft.

The door plug was discovered by a Portland school teacher who found it in his backyard.

It’s led to a major investigation, as other airlines began to ground the aircraft type.

Who makes this panel?

The fuselage for the Boeing 737 is manufactured by Spirit AeroSystems, a Kansas-based company that separated from Boeing in 2005.

Spirit is one of two suppliers that make the plug doors on the MAX 9, but Boeing also plays a critical role in the plug installation process.

Boeing has come under scrutiny in recent years over its manufacturing processes.

Why is this panel there?

The 737 MAX 9 is currently Boeing’s largest single-aisle aircraft, capable of seating up to 220 people.

It includes an optional extra door to allow for the approved number of evacuation paths whenever carriers choose to install the maximum number of seats.

Planes that do not opt for additional seating can replace that door with a panel, or plug.