The investigation into the mid-flight blowout of a Boeing 737 Max 9 continues as investigators work to determine the cause of the incident. National Transportation Safety Board (NTSB) Chairwoman Jennifer Homendy provided an update on the investigation during a closed-door briefing for lawmakers on Wednesday.

The focus of the investigation is on a door plug that blew off Alaska Airlines flight 1282 on January 5, causing a significant hole in the side of the aircraft. Investigators are currently examining whether bolts were properly installed on the door plug. However, Chairwoman Homendy clarified that the investigation is not solely focused on the bolts, and her teams are collecting extensive records on the assembly and journey of the door plug from Malaysia to factories in Kansas and Washington.

The NTSB's lab is currently scrutinizing the plug but has not yet started disassembling it. Next week, NTSB officials will begin the process of pulling the door plug apart for further examination.

In response to this incident, the Federal Aviation Administration (FAA) announced on Wednesday that it is expanding its probe into the quality control of the Boeing 737 Max 9. The FAA will now investigate contractor Spirit AeroSystems, which builds the fuselage of the aircraft. Last week, the FAA had initiated an investigation into Boeing's quality control.

The FAA investigation will focus on whether Boeing failed to ensure that its completed products adhered to approved designs and met safety regulations. Boeing has expressed its full cooperation with the FAA and the NTSB in their investigations.

Currently, all 171 Boeing 737 Max 9s in the United States remain grounded. The FAA has completed preliminary inspections on 40 of these airplanes and is reviewing the data obtained. Once the FAA approves an inspection and maintenance process, it will be required on every grounded 737-9 Max before resuming future operations.

To restore confidence in its manufacturing, Boeing has announced its decision to allow airlines access to its factories, as well as those of Spirit AeroSystems. This move aims to provide transparency and demonstrate commitment to addressing any concerns.

Last year, shareholders of Spirit AeroSystems filed a federal lawsuit against the company, alleging "widespread and sustained quality failures" in its products. The lawsuit claims that these quality failures were due to a company culture that prioritized production numbers over product quality. Spirit AeroSystems strongly denies the claims and intends to defend against them.

In previous instances, Spirit AeroSystems had identified production issues on certain models of the Boeing 737, but both Spirit and Boeing clarified that these issues did not pose immediate safety concerns.

The ongoing investigation, as well as the FAA's expanded probe, highlights the commitment to ensuring the safety and quality of Boeing aircraft. As further developments emerge, aviation authorities and industry stakeholders will closely monitor the progress and take necessary actions to prevent such incidents in the future.