The Federal Aviation Administration (FAA) announced on Thursday a formal investigation into the midair blowout of a “door plug” on a Boeing 737 MAX 9 jetliner last Friday.

The Alaska Airlines flight from Portland, Oregon to Ontario, California was forced to turn around for an emergency landing after the door plug blew out at an altitude of 16,000 feet as the plane was still climbing. Miraculously, no one was seriously injured in the failure, which resulted in a rapid loss of cabin pressure and caused oxygen masks to drop, while passengers and crew members endured a terrifying descent and landing.

In a press statement accompanying the official FAA letter to Boeing, the US agency wrote, “This incident should have never happened and it cannot happen again. FAA formally notified Boeing that it is conducting an investigation to 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.”

In its letter, the FAA said it had been “notified of additional discrepancies on other Boeing 737-9 airplanes,” although it did not disclose details of these issues. The rest of the letter contains boilerplate text about the Title 14 Code of Federal Regulations and Boeing’s noncompliance with its own inspection and testing requirements.

Airline experts have said that by formally notifying Boeing of the investigation, the FAA is setting into motion a process that “could” result in enforcement actions such as “fines” and other measures that require the company to fix the problems identified.

The FAA letter went on to state that the jet manufacturer “may have failed to ensure its completed products conformed to its approved design and were in a condition for safe operation in accordance with quality system inspection and test procedures.”

Door plugs are used by Boeing to seal unused plane exits on certain models. The Model 737 MAX 9 can hold anywhere from 162 to 220 seats, depending on the airline company’s requirements. If the airline uses the maximum number of seats, an additional exit is required between the wings and the rear doors.

The door plugs are only visible from outside the plane. On the inside, by row 26 of Alaska Airlines Flight 1282, the door plug is hidden by the cabin wall. According to a diagram released by the National Transportation Safety Board (NTSB), the door plug is attached to the plane by a series of bolts, cables and stop pads.

The NTSB is investigating why the Alaska Airlines door panel separated from the plane. It is currently unclear as to whether the bolts that hold the panel in place were either missing or installed incorrectly. Both Alaska Airlines and United Airlines, which operate the majority of the MAX 9s in the US, said on Monday they had discovered loose hardware on the panel when conducting preliminary inspections on their planes.

There are 171 MAX 9s grounded in the US as a result of the investigations underway, including 65 at Alaska Airlines and 79 at United Airlines, which cancelled 8 percent of its total schedule, or 225 flights, on Tuesday. Other airlines with grounded MAX 9 models include Copa Airlines (21), Aeromexico (19) and Turkish Airlines (5).

Two days after the blowout, Boeing CEO Dave Calhoun announced he would cancel a two-day leadership summit and instead hold an all-employee safety meeting from the 737 factory in Renton, Washington on Tuesday. Calhoun also sent out a company-wide memo to staff, which made carefully vetted and pro forma comments, such as, “When it comes to the safety of our products and services, every decision and every action matters.”

At the employee meeting on Tuesday, Calhoun said the company was “acknowledging our mistake,” and added, “We’re going to approach it with 100 percent and complete transparency every step of the way. We are going to work with the NTSB, who is investigating the accident itself, to find out what the cause is.”

Calhoun came to Boeing from the private equity division of The Blackstone Group, which has $42.5 billion in assets under management. After serving on the Boeing Board of Directors since 2009, Calhoun was brought on as CEO after Dennis Muilenburg was fired following the catastrophic crashes of 737 MAX 8 planes in Indonesia in 2018 and Ethiopia in 2019, which killed a total of 346 passengers and crew.

Between those disasters and now, following an initial flurry of corporate hand-wringing and regulatory noise-making in Washington, D.C., Boeing has been repeatedly given a pass on its shirking of safety requirements in the interests of speeding production, cutting costs and boosting profits. Throughout the process, Senator Maria Cantwell, Democrat from Boeing’s home state of Washington and a recipient of $200,000 in campaign contributions from the airplane manufacturer’s political action committee (PAC), has assisted the company in avoiding government safety requirements and deadlines.

As chair of the Senate Commerce Committee, Cantwell worked with other Democrats in 2022 to provide Boeing with an exemption to the requirement that airlines upgrade their crew alert systems on all MAX airplanes. Over the opposition of 921 friends and family members of the Ethiopian Airlines Flight 302 crash, Boeing “advocated and received an FAA approved exception from the flight crew alert rule for the MAX 8 and 9 aircraft,” according to a letter sent to Democratic Party leaders. The Senate overlooked the letter from past victims of Boeing’s malfeasance and passed the exemption.

Less than a month before Friday’s blowout failure, former employees at Boeing’s subcontractor, Spirit AeroSystems, filed a class action lawsuit documenting that they repeatedly warned corporate officials about safety problems and were told to falsify records.

One of the employees at Spirit, which is apparently the manufacturer of the door plug that blew off of the Alaska Airlines flight over Portland, told company officials about an “excessive amount of defects.” Among the known defects that were ignored by Spirit were “mis-drilled holes in the aft pressure bulkhead.” These errors were the “worst findings” ever made by a lead Core Quality Auditor.

According to the lawsuit, another employee “struggled with Spirit’s culture, which placed an emphasis on pushing out product over quality,” and he later told a fellow employee that “he believed it was just a matter of time until a major defect escaped to a customer.”

Representative Ron Estes, Republican from Kansas—and one of the top recipients of Spirit-affiliated campaign donations—has complained that the FAA’s grounding of the 737 MAX 9 jets has “negative repercussions.” He warned that “until recertification [of the Boeing 737 MAX] is complete,” Spirit and its workers would suffer.

According to US lobbying disclosures, over the last four years, Boeing and Spirit PACs and individual employees have spent more than $65 million on lobbying and federal campaign contributions.