Boeing's manufacturing processes are under investigation after the Alaska Airlines blowout, the FAA says

• The FAA is investigating Boeing's manufacturing practices and production lines.
• It comes after the Alaska Airlines blowout led to 171 Boeing 737 Max 9 jets being grounded.

The Federal Aviation Administration announced Wednesday that it is investigating Boeing's manufacturing practices and production lines.

This includes processes involving Spirit AeroSystems, the Kansas-based company that builds the fuselages and other parts of Boeing planes.

The FAA said it's also "bolstering its oversight of Boeing, and examining potential system change."

The agency grounded 171 Boeing 737 Max 9 jets with a door plug following the Alaska Airlines blowout.

The jet's door plug — which covers a deactivated emergency exit that's only operational in configurations with more seats — came off the plane at an altitude of 16,000 feet.

Oxygen masks were deployed but nobody was seriously injured as the flight returned to Portland International Airport 20 minutes after takeoff.

The incident has renewed scrutiny of Boeing's quality-control processes.

After the National Transportation Safety Board recovered the Alaska Airlines door plug, investigators found that four bolts attaching it to the fuselage were missing. Further testing is underway to determine whether the bolts were ever installed.

Both Alaska Airlines and United Airlines also found loose hardware on some of their aircraft shortly after the incident.

Boeing's initial inspection instructions to operators were revised after feedback, extending the disruption which has canceled flights for over a week. The FAA's final approval of the inspection and maintenance process is still pending.

The FAA formally notified Boeing last Thursday that it is investigating whether it failed to ensure 737 Max 9 jets were in a safe condition and complied with regulations.

On Monday, Boeing announced new measures to improve its quality-control system. They include an additional layer of inspections through the build process, opening the factory to operators for inspections, and an independent assessment to suggest changes.

Boeing declined to comment when contacted by Business Insider.