The US Federal Aviation Administration (FAA) announced an investigation into Boeing’s quality control on Thursday, after a near-catastrophic incident last week involving an Alaska Airlines-operated 737 MAX. “This incident should have never happened and it cannot happen again,” the FAA said in a statement, after a panel known as a “door plug” blew out of the plane over the western US state of Oregon. The Alaska Airlines jet successfully executed an emergency landing with no fatalities or major injuries. US regulators have since grounded 171 737 MAX 9 planes with the same configuration as the jet involved in last Friday’s incident. The FAA said Thursday 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.” “Boeing’s manufacturing practices need to comply with the high safety standards they’re legally accountable to meet,” the FAA added in a statement. It said that safety, rather than speed, would decide the timeline for returning the 737-9 MAX to service. Earlier this week, Boeing Chief Executive Dave Calhoun took responsibility for the incident, vowing “complete transparency” as the aviation giant tries to pivot from its latest crisis. The affected door plug panel is used to fill an unneeded emergency exit in planes, and National Transportation Safety Board investigators have suggested that the part was not affixed adequately. Both United Airlines and Alaska Airlines reported finding loose hardware on some of their Boeing 737 MAX 9 planes during preliminary inspections undertaken since Friday’s incident.