Investigators at the National Transportation Safety Board say they have recovered the "door plug" that blew off a Boeing 737 Max 9 airplane Friday night — and hope it will yield important clues about why this section of fuselage detached from the rest of the plane at 16,000 feet.

"We're very fortunate they have found the plug itself," said John Cox, a former pilot and safety consultant, in an interview with NPR's Morning Edition.

Investigators "will want to look at everything on the door," NTSB Chair Jennifer Homendy said during a briefing Sunday night.

Alaska Airlines flight 1282 quickly returned to Portland, Ore., after the blowout. No one was seriously injured. But the incident has raised concerns about the Boeing 737 Max 9, a larger cousin of the two 737 Max 8 jets that crashed in 2018 and 2019, killing a total of 346 people. It's also prompting new questions about the door plug system itself, and whether it's still safe to fly.

What is a door plug?

The door plug is not really a door at all. It's a component that's designed to fill a hole in the fuselage of the plane where an optional emergency exit would also fit.

Planes that carry more than about 200 passengers require more emergency exits to comply with safety regulations, while airplanes that carry fewer passengers can be fitted with the door plug instead.

Under ordinary circumstances, most passengers wouldn't notice the door plug at all because it looks similar to a regular window.

Boeing has been using the design for more than a decade without any major incidents, said Cox, who is now a consultant with the company Safety Operating Systems.

"This design has been in use for a number of years and it's not proven to be problematic at all," Cox said. Door plugs are used on the Boeing 737-900ER, a predecessor of the Max planes, he said, as well as the Max series.

The Alaska Airlines plane had just been delivered on October 31st, according to the NTSB. An auto-pressurization failure light had illuminated in the plane's cockpit three times in prior weeks, Homendy said. Alaska Airlines put a restriction in place that prevented the plane from flying over water to Hawaii, so that it could return more easily to an airport in case of emergency.

What are investigators looking for?

"We know what happened. We don't know fully why," Cox said. "And then the follow up question, of course, is what do we need to do to prevent it from happening again?"

The door plug is held in place by four bolts. And investigators say the condition of those bolts may be telling.

"Are the four bolts there? Are the nuts there? Was there deformation or bending of the bolts, of the holes?," Cox said. "All of those things they're going to look at to try to understand the forces that resulted in this plug leaving the airplane."

Investigators at the NTSB will want to examine both the door and the components of the plane where it was attached.

"We have a lot of ability in our lab with our microscopes to really look at some of the components more in depth," Homendy said on Sunday night, "to look at witness marks, to look at any paint transfer, what shape the door was in when found. That can tell them a lot about what occurred."

The door plug, like the rest of the Max 9 fuselage, is manufactured by Spirit AeroSystems, a Boeing supplier based in Wichita, Kansas.

"We are grateful the Alaska Airlines crew performed the appropriate procedures to land the airplane with all passengers and crew safe," the company said in a statement.

"At Spirit AeroSystems, our primary focus is the quality and product integrity of the aircraft structures we deliver. Spirit is a committed partner with Boeing on the 737 program, and we continue to work together with them on this matter," the statement said.

The company's previous CEO stepped down in October amid problems with production, and was replaced by a former Boeing executive.

How will this affect air travel?

The Federal Aviation Administration said Monday that carriers can begin inspections of about 170 Boeing Max 9 planes which have been grounded after the incident on Friday night.

"The FAA's priority is always keeping Americans safe," the agency said in a statement. "Boeing 737-9 aircraft will remain grounded until operators complete enhanced inspections which include both left and right cabin door exit plugs, door components, and fasteners."

Operators must also complete any corrective actions based on the findings from those inspections, the agency said. The FAA previously said that inspections would take about four to eight hours per aircraft.

"We agree with and fully support the FAA's decision to require immediate inspections of 737-9 MAX airplanes with the same configuration as the affected airplane," said Boeing commercial airlines president and CEO Stan Deal and safety officer Mike Delaney in an email to the company's employees.

"The assembly to be inspected is not found on other members of the 737 MAX family," they noted.

The FAA ordered the grounding of 737 Max 9 planes in certain configurations that have the door plug. But other planes that have door plugs are still flying, including the 737-900ER as well as other 737 Max 8 planes that also have the door plug.

Regulators in Europe say the order will have no immediate impact on airlines there.

"The 737-9 aircraft operating in Europe do not have this configuration and are therefore not grounded," the European Union Aviation Safety Agency said in a statement today, "and can continue to operate normally."

European airlines including Ryanair fly the Boeing 737 Max 9, but they are fitted with emergency exits instead of door plugs.

"Right now, this still looks like it's a one off," Cox said. "It's just something that happened to this airplane." But the ongoing problems with the Max series are yet another blow to Boeing's reputation, he said. "An operator around the world is going to look at this and say, okay, if we buy the Max, are we buying a problem?"