The Federal Aviation Administration said it is conducting an investigation of Boeing after a defective door plug fell out of an Alaska Airlines plane last week, forcing an emergency landing.
"This incident should have never happened and it cannot happen again," the FAA said in a statement, noting it has formally notified Boeing of the probe.
The door plug for the fuselage of a Boeing 737 Max 9 fell off a few minutes after Alaska Airlines Flight 1282 took off from Portland International Airport on Friday, depressurizing the cabin and exposing passengers to open air thousands of feet above ground. Passengers captured footage showing a hole where the door plug came loose.
Boeing said in a statement, "We will cooperate fully and transparently with the FAA and the NTSB on their investigations."
The FAA said its 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."
The Boeing probe is a result of the door plug incident and "additional discrepancies," the FAA said.
"Boeing's manufacturing practices need to comply with the high safety standards they're legally accountable to meet," the agency said.
Every Boeing 737 Max 9 with a plug door will remain grounded until the administration determines that each can safely return to operation, the FAA said Tuesday. The pause affects about 171 planes worldwide.
"The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service," the FAA said Thursday.
The Alaska Airlines emergency incident is being investigated by the National Transportation Safety Board.
Boeing CEO Dave Calhoun told employees on Tuesday that the company is "going to approach" the midair emergency by starting with an acknowledgment of "our mistake."
"We're going to approach it with 100% and complete transparency every step of the way," Calhoun said during a meeting with employees at the 737 production facility in Renton, Washington. "We are going to work with the NTSB who is investigating the accident itself to find out what the cause is."
The focus of the NTSB investigation is on the single aircraft, but could be broadened as more is learned, board Chief Jennifer Homendy said.
"However, at some point, we may need to go broader. But right now we have to figure out how this occurred with this aircraft," Homendy said Tuesday on ABC News' "Good Morning America."
The fittings at the top of the door plug fractured, Homendy said. The NTSB examination has shown that those fittings were fractured, allowing the plug door to move upward and outward, she said.
"We don't know if the bolts were loose. We don't know if bolts were in there fractured or possibly the bolts weren't there at all," she said. "We have to determine that back in our laboratory."
On Monday, United Airlines said it had found loose bolts on its 737 Max 9 fleet during inspections ordered after Friday's incident involving an Alaska Airlines flight.
ABC News' Kevin Shalvey contributed to this report.