(NEWSnet/AP) – Boeing has discovered another problem in some of its 737 model fuselages that may delay deliveries of about 50 aircraft.

Improperly drilled holes were discovered by Spirit AeroSystems, a major supplier that provides Boeing with fuselages, according to a letter from Stan Deal, CEO of Boeing's commercial airplanes division.

“While this potential condition is not an immediate safety issue and all 737s can continue operating safely, we currently believe we will have to perform rework on about 50 undelivered planes,” Deal said in a letter to employees that was shared Monday.

Boeing and Spirit AeroSystems are facing intense scrutiny over the quality of their work after an Alaska Airlines 737 Max 9 was forced to make an emergency landing on Jan. 5 when a panel called a door plug blew out of the side of the plane shortly after takeoff from Portland, Oregon.

The NTSB is investigating the accident, while the Federal Aviation Administration investigates whether Boeing and its suppliers followed quality-control procedures.

Alaska Airlines and United Airlines, both of which fly the Max 9, reported finding loose hardware in door plugs of other planes they inspected after the accident. The FAA grounded all Max 9s in the U.S. the day after the blowout. Two weeks later, the agency approved the inspection and maintenance process to return the planes to flying.

Alaska Airlines and United Airlines have begun returning some of their planes to service.

But United Airlines CEO Scott Kirby said last month that the carrier will consider alternative aircraft in the future.

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