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The US Federal Aviation Administration (FAA) has formally notified Boeing in a letter that it is conducting an investigation into the American aircraft maker's quality control after the January 5 Alaska Airlines in-flight blowout incident.

In a statement released on Thursday, the aviation watchdog said the investigation will "determine if Boeing failed to ensure completed products conformed to its approved design and were in a condition for safe operation in compliance with FAA regulations", reports Xinhua news agency

The investigation is the result of the incident on a Boeing Model 737-9 Maz where it lost a "plug" type passenger door and additional discrepancies, the statement said.

"Boeing's manufacturing practices need to comply with the high safety standards they're legally accountable to meet. This incident should have never happened and it cannot happen again," FAA stressed.

The mid-cabin door plugs on a Boeing 737 Max 9 operated by Alaska Airlines became dislodged from the aircraft following an abrupt depressurisation shortly after departure on January 5.

As a result, a piece of the fuselage was expelled at an altitude of 16,000 feet.

The plane, bound for Ontario, California, executed an emergency landing in Portland just 20 minutes after takeoff.

On January 6, the FAA temporarily grounded 171 737 Max 9 planes on an international level with door plugs pending inspections.

Two airlines that operate Boeing 737 Max 9 in the US -- Alaska Airlines and United Airlines -- later found either loose hardware or bolts in the assembly of door plugs on their aircraft.

The FAA's investigation is separate from an investigation conducted by the National Transportation Safety Board specifically into the incident itself, local media reported.