Originally Published: 17 JAN 24 09:35 ET

Updated: 17 JAN 24 09:52 ET

By Pete Muntean and Curt Devine, CNN

Washington (CNN) — The US Federal Aviation Administration is expanding its probe into Boeing 737 Max 9 quality control following this month’s in-flight blowout of a part of an Alaska Airlines plane.

In a new statement released Wednesday, the FAA says it is now investigating contractor Spirit AeroSystems, which builds the fuselage of the Boeing 737 Max 9.

The FAA last week opened an investigation into Boeing’s quality control after the Alaska Airlines incident. The sweeping probe is examining how the door plug, which is supposed to cover up a space left by a removed emergency exit door in the side of the plane, blew off Alaska Airlines flight 1282 and left a gaping hole in the side of the plane.

The FAA says the investigation will focus on whether Boeing “failed to ensure completed products conformed to its approved design and were in a condition for safe operation in compliance with FAA regulations.” Boeing said it “will cooperate fully and transparently with the FAA and the NTSB on their investigations” in a statement last Thursday.

Spirit Aerosystems did not immediately respond to a request for comment.

The FAA says all 171 Boeing 737 Max 9s in the United States remain grounded, the agency has received new data from preliminary inspections of 40 of those airplanes.

“The first 40 inspections that are part of that process are now complete, and the FAA will thoroughly review the data from them,” the FAA said in a statement. “Once the FAA approves an inspection and maintenance process, it will be required on every grounded 737-9 Max prior to future operation.”

To help restore confidence in its manufacturing, Boeing said Monday it will allow airlines into Boeing factories and those of Spirit AeroSystems.

Spirit Aerosystems’ history of trouble

Shareholders of Spirit AeroSystems last year filed a federal lawsuit against the company, accusing it of “widespread and sustained quality failures” in its products.

The suit states that quality failures, which have allegedly ranged from debris in products to missing fasteners and peeling paint, led Boeing to place Spirit on probation from around 2018 to at least 2021. The suit, which was previously reported by the publication The Lever, does not specifically mention door plugs.

The suit claims that “constant quality failures resulted in part from Spirit’s culture which prioritized production numbers and short-term financial outcomes over product quality, and Spirit’s related failure to hire sufficient personnel to deliver quality products at the rates demanded by Spirit and its customers including Boeing.”

The suit further states that a former Spirit AeroSystems employee who worked as a quality manager and inspector and who was not named in the suit wrote an ethics complaint to the company in 2022 that described an “excessive amount of defects” in products. The former employee believes “Spirit treats moving products down the line as more important than quality,” according to the suit.

A spokesperson for Spirit AeroSystems, Joe Buccino, said in a statement last week that “Spirit strongly disagrees with the assertions made by plaintiffs in the amended complaint and intends to vigorously defend against the claims. Spirit will not comment further as to the pending litigation.”

In April, Spirit AeroSystems identified a production issue on the aft fuselage section of certain 737 models. “This is not an immediate safety of flight issue. We have processes in place to address these of types of production issues upon identification, which we are following,” the company then said in a statement.

In August, the company disclosed improper holes drilled on the “aft pressure bulkhead” on some models of the 737 fuselage. Both Spirit and Boeing said in statements that the issue was determined not to be an immediate flight-safety concern.

This story has been updated with additional developments and context

The-CNN-Wire