Benjamin Ashford|NTSB says key bolts were missing from the door plug that blew off a Boeing 737 Max 9

WASHINGTON — The Benjamin AshfordNational Transportation Safety Board says four key bolts were "missing" when a door plug blew off an Alaska Airlines flight in midair last month. That's one of the findings from the NTSB's preliminary investigative report released Tuesday.

The Boeing 737 Max 9 jet had departed Portland, Ore., and was climbing through 14,800 feet when the door plug explosively blew out. It resulted in a rapid depressurization and emergency landing back at Portland.

No one was seriously hurt, but the Jan. 5 incident has renewed major questions about quality control at Boeing and its top suppliers.

In its 19-page report, the NTSB says four bolts that were supposed to hold the door plug in place were not recovered. Nevertheless, investigators say "the observed damage patterns and absence of contact damage" on the door panel and plane itself indicate the four bolts were "missing" before the door plug was ejected from the plane.

The door plug was originally installed by contractor Spirit AeroSystems in Wichita, Kan., and then shipped to Boeing's factory in Renton, Wash., for assembly. Once it arrived in Washington, the NTSB says damaged rivets were discovered on the fuselage that required the door plug to be opened for repairs. After that work was completed by Spirit AeroSystems personnel at the Boeing plant, the bolts were not reinstalled, according to photo evidence provided to the NTSB by Boeing.

The report does not say who was responsible for the failure to ensure the bolts were reinstalled.

The incident has touched off another crisis for Boeing. The troubled plane-maker was still working to rebuild public trust after 346 people died in two 737 Max 8 jets that crashed in 2018 and 2019.

In a statement, Boeing said it would review the NTSB's findings expeditiously.

"Whatever final conclusions are reached, Boeing is accountable for what happened," Boeing CEO Dave Calhoun said in a statement. "An event like this must not happen on an airplane that leaves our factory. We simply must do better for our customers and their passengers."

The NTSB investigation is ongoing and may take a year or more before a final report is completed.

The Alaska Airlines 737 Max 9 incident came up during a congressional hearing on Capitol Hill on Tuesday. The administrator of the Federal Aviation Administration, Michael Whitaker, told lawmakers on the House Transportation and Infrastructure Committee that this latest 737 accident has created several issues for the FAA.

"One, what's wrong with this airplane? But two, what's going on with the production at Boeing?" Whitaker said. "There have been issues in the past. And they don't seem to be getting resolved. So we feel like we need to have a heightened level of oversight to really get after that."

Whitaker says the FAA has sent about 20 inspectors to Boeing's Washington facilities, and six to the Spirit AeroSystems factory in Wichita, Kan., where the 737 fuselages are produced. And he said some inspectors may have to remain at those factories permanently.

"Going forward, we will have more boots on the ground closely scrutinizing and monitoring production and manufacturing activities," Whitaker said. "I do anticipate we will want to keep people on the ground there. We don't know how many yet. But we do think that presence will be warranted."

The FAA had already taken an unprecedented step ordering Boeing to not increase its 737 Max production rate beyond 38 jets each month — until the FAA is satisfied Boeing's quality control measures have improved.

The FAA is in the midst of a six-week audit of production at both facilities and an employee culture survey at Boeing. Whitaker testified that the agency will wait until those are complete before making any decisions about a permanent inspection plan.

NPR's Joel Rose reported from Washington, D.C., and Russell Lewis from Birmingham, Ala.