NTSB investigating if Alaska Airlines door found in Portland backyard was bolted properly

Federal investigators say they are examining a door plug that blew out midair from an Alaska Airlines flight on Friday after the panel was found in a Portland, Oregon, backyard by a school teacher.

Investigators said at a Monday news briefing that the fittings on the top of the door plug fractured, causing the door plug to move upward and outward. It remains unclear if the panel was properly bolted.

The door panel, which covers an extra emergency exit that is only operable on planes with the maximum capacity, blew off Alaska Airlines Flight 1282, which was at 16,000 feet and climbing to cruising altitude after departing Portland, Oregon, for Ontario, California. The loss of the panel caused the depressurization of the cabin and the plane returned safely to Portland with no serious injuries reported. 

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The panel was discovered on Sunday by a Portland schoolteacher who found it in his backyard intact in a tree’s lower branches.

"We have not yet recovered the four bolts that restrain it from his vertical movement, and we have not yet determined if they existed there," Clint Crookshanks, of the National Transportation Safety Board (NTSB) said at a news conference Monday. 

"That will be determined when we take the plug to our lab. We don’t know if there were bolts there, or if they are just missing and departed when the door plug departed."

Door plugs are sometimes installed on the fuselage by a manufacturer instead of an emergency exit door, depending on the number of emergency exits required. The two seats nearest the panel were empty at the time of the frightening incident, while a cell phone and a passenger’s shirt were sucked out the hole in the cabin.

NTSB Chair Jennifer Homendy said that locating the door was critical to the investigation and had previously described it as the "key missing component."

"We’re able to look at all the components on this door plug, all the fittings, all – any sort of structures that may remain," Homendy said.

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Homedy added that materials engineers will be looking at all aspects of the door including bolts, washer, nuts and other components. 

"We don’t know if the bolts were loose, we don’t know if bolts were in there fractured, or possibly the bolts weren’t there at all," Homendy told "Good Morning America" on Tuesday. "We have to determine that back in our laboratory."

The news comes after United Airlines announced Monday that it found loose bolts on its Boeing 737 MAX 9 planes.

It followed revelations that Boeing and the Federal Aviation Administration (FAA) in late December sent a warning to 737 MAX operators to inspect their aircraft for a loose bolt that could impact its rudder control system.

United Airlines and Alaska Airlines, the two U.S. air carriers that use the 737 MAX 9, have grounded their fleets to inspect their aircraft while the FAA and NTSB investigate the incident which resulted in hundreds of flights getting canceled. 

Homedy also said that the plane’s cockpit voice recorder was "completely overwritten," striking a blow for investigators. 

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"If that communication is not recorded, that is, unfortunately, a loss for [the NTSB], and a loss for the FAA and a loss for safety because that information is key not just for our investigation, but for improving aviation safety," Homendy said Monday.

Regarding the plane in Friday’s incident, Homedy said that the auto pressurization fail light came on three times in the past month, and Alaska Airlines restricted it from flying over the ocean in case it needed to return to an airport, although she said it is not clear if those incidents are related to the door panel coming off.

"At this time, we have no indications whatsoever that this correlated in any way to the expulsion of the door plug and the rapid decompression," Homendy said, adding that the problem was checked by maintenance workers. 

Fox News’ Eric Revell, Andrea Vacchiano and Grady Trimble contributed to this report.