The regulator said the company’s “manufacturing practices need to comply with the high safety standards they’re legally accountable to meet.” The investigation is a result of the midair blowout of a so-called door plug on a 737 Max 9 jet operated by Alaska Airlines on Jan. 5.

“This incident should have never happened and it cannot happen again,” the agency said in a statement.

The probe compounds the troubles unfolding at Boeing after the fuselage panel blew off Alaska Airlines Flight 1282 shortly after takeoff from Portland, Oregon, with 177 aboard. The agency said it aims to determine 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.”

In a letter to Boeing laying out its decision, the FAA said it was notified of “additional discrepancies” on other 737 Max 9 aircraft after the accident, though it didn’t provide details. Boeing said it will “cooperate fully and transparently” with the investigations.

The FAA’s move heightens the stakes for Boeing, which is already facing heavy scrutiny over a series of quality issues across its aircraft programs. Regulators grounded 171 of the 737 Max 9 jets in operation after the Alaska accident to allow for inspections, prompting Max 9 operators to cancel hundreds of flights.

Falling Stock

United Airlines Holdings Inc., the largest operators of the model, found loose bolts on several jets during preliminary inspections. Alaska Air Group Inc. has said “loose hardware was visible” on some of its other Max 9 planes during examinations that followed the accident. United Airlines didn’t immediately comment on the FAA’s latest action. Alaska Airlines didn’t respond to a request seeking comment.

Boeing shares fell 1% as of 1:10 p.m. in New York. The stock has declined 14% this year amid the near-disaster and Max 9 grounding.

The FAA’s decision comes after US Transportation Secretary Pete Buttigieg on Wednesday hinted that the Max 9 wouldn’t be rushed back into service. At a transportation conference in Washington, he said the aircraft would remain grounded until regulators deem them safe to fly, which he called the “only consideration.”

“The FAA is showing that they are being very aware and executing their oversight in a transparent way, but are also putting Boeing on notice that they want some answers,” said John Cox, chief executive of Safety Operating Systems, an aviation consulting firm. “They want to be sure all the manufacturing processes and procedures are followed to the letter.”

In the accident last week, an almost-new 737 Max 9 saw a large panel of the left-side fuselage eject during flight, leaving passengers exposed to a gaping hole. Nobody was seriously injured and the plane returned safely, but pressure has built on Boeing and its senior management to explain the defect.

In a companywide address on Wednesday, Boeing Chief Executive Officer Dave Calhoun said the company acknowledged its “mistake” and stressed that workers must elevate safety as their top priority.

The FAA gave Boeing 10 days to provide evidence, including the root cause of the panel’s blowout, products affected, and what it’s doing to prevent a recurrence.