Missing bolts likely contributed to an emergency blowout aboard a Boeing 737 MAX 9 during an Alaska Airlines flight last month, according to the National Transportation Safety Board's (NTSB) preliminary findings on the incident released on Tuesday.

Why it matters: The findings shed light on the near catastrophe that's reignited scrutiny on Boeing and its 737 MAX jets, which were previously involved in two crashes that killed 346 people.

• The crashes, one in 2018 and another in 2019, led to the resignation of top Boeing executives, a criminal investigation and a massive financial hit for one the largest aerospace manufacturers in the world.
• Alaska Airlines flight 1282 suffered an in-flight emergency on Jan. 5 after a door plug in the airline's Boeing 737 MAX 9 jet was torn from the vehicle's fuselage.

Catch up fast: Passengers and crew were unharmed, but the near disaster prompted the FAA to order operators to ground and inspect all 737 MAX 9 jets.

• Most of the jets have returned to service, but the inspections revealed quality control issues, such as loose bolts.

Details: The NTSB said four bolts are normally used to prevent the 737 MAX's mid-exit door plug from moving vertically while flying.

• In the case of the 737 MAX involved in the Alaska Airlines incident, it said a damage assessment indicated "that the four bolts that prevent upward movement of the [mid exit door] plug were missing."
• The board said records show the plane went under repairs to replace damaged rivets near the left exit door that blew off during the accident.
• To complete the repairs, workers had to remove bolts and open the affected door plug.

Photos of the door plug provided by Boeing after the repairs were completed in September 2023 showed evidence that the plug was reclosed "with no retention hardware (bolts) in the three visible locations," it said.

• It's unclear if the fourth bolt was also missing when the photo was taken, as its location was covered by insulation.
• The NTSB said the rivet repairs were carried out by workers from Spirit AeroSystems, which produces the fuselage for the MAX jets.
• However, it's unclear if Boeing or Spirit AeroSystems personnel were responsible for opening and reclosing the door plug.
• The NTSB report doesn't indicate which company is at fault for the malfunction, though it notes an investigation is ongoing to determine which reclosed the door plug.

What they're saying: Regardless of the NTSB's ultimate findings, Boeing is accountable for what happened, its president and CEO Dave Calhoun said in a statement after the preliminary report was released.

• "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," Calhoun said.
• He said the company has implemented a new plan to improve quality control throughout the 737 production process, including adding more inspections further into the supply chain and bringing in independent aircraft assessments.

Worth noting: Federal Aviation Administration (FAA) administrator Michael Whitaker vowed before a House subcommittee on Tuesday that he would hold Boeing accountable for any noncompliance that contributed to the Alaska Airlines incident.

• Because of the midflight blowout, Boeing is again facing regulatory and legal repercussions over 737 MAX malfunctions just as the company was attempting to revive its reputation and quell safety concerns.

The big picture: After the blowout, the FAA said it would begin on-site inspections of Boeing facilities as part of an audit of the MAX production line to ensure the company is complying with quality procedures.

• The inspections would also apply to Spirit AeroSystems facilities.
• Whitaker also said Tuesday the agency would not give Boeing additional manufacturing expansions for the MAX until quality control concerns are resolved.

Read the full NTSB preliminary report below.

Go deeper: Boeing reworking 50 undelivered 737 MAX jets over fuselage issue

Editor's note: This article has been updated with a statement from Boeing.