A panel that blew off a Boeing 737 Max during an Alaska Airlines flight was displayed for the media Tuesday at the National Transportation Safety Board’s laboratory in Washington, D.C., and will be a key part of a two-day investigative hearing set for next week.

Federal investigators have been examining the mid exit door plug since not long after it blew off during a Jan. 5 flight, when the plane was at 16,000 feet and climbing toward cruise altitude. It was discovered in the backyard of a home near Portland, Oregon.

A preliminary report from the NTSB said that four bolts used to help secure the part to the jetliner were missing when the plane rolled out of a Boeing factory near Seattle, before it was delivered to Alaska Airlines.

Door plugs are panels that seal holes left for extra doors when the number of seats is not enough to trigger a requirement for more emergency-evacuation exits. From inside the plane, they look like a regular window.

They are only intended to be opened for maintenance and inspections. The one on the Alaska Airlines plane was installed at a Spirit AeroSystems plant in Wichita, Kansas, but was removed at the Boeing plant so workers could fix damaged rivets.

The NTSB says it will use information from the hearing on Tuesday and Wednesday to complete its investigation of the blowout and make safety recommendations.

(AP)