The Federal Aviation Administration delivered a letter to Boeing Thursday in the wake of the ongoing 737 Max 9 grounding, saying that it is opening an investigation into the manufacturer.

“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. This investigation is a result of an incident on a Boeing Model 737-9 MAX where it lost a ‘plug’ type passenger door and additional discrepancies. Boeing’s manufacturing practices need to comply with the high safety standards they’re legally accountable to meet,” a statement the agency released with the letter said. “The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service.”

According to the FAA, Boeing has 10 days to respond to the notice, and the agency is seeking information regarding “the root cause of the encountered condition(s), products/articles affected, service impacts, the extent of any immediate/long-term action taken to correct and preclude its recurrence, and any mitigating circumstances which you believe may be relevant to this case.”

In a statement, Boeing’s CEO Dave Calhoun said the company plans to fully cooperate with the investigation.

During a company town hall earlier this week, he said that trustworthiness is an important value to Boeing.

“We’re going to approach it with 100% complete transparency every step of the way,” he said. “We’re going to have to demonstrate it by our actions, by our willingness to work directly and transparently with (our customers). And to make sure they understand that every airplane that Boeing has its name on that’s in the sky is in fact safe.”

Cruising Altitude: I’ve covered Boeing’s 737 MAX for years. A quick rundown of the issues

Some 737 Max 9 jets remain grounded as the FAA works to iron out specific requirements for the hardware inspections that airlines will have to carry out before they are returned to service. The grounding order has been in effect since Saturday, following an explosive decompression on an Alaska Airlines flight that resulted from a piece of the fuselage ripping away in flight.

Zach Wichter is a travel reporter for USA TODAY based in New York. You can reach him at zwichter@usatoday.com

This article originally appeared on USA TODAY: FAA is investigating Boeing as 737 Max 9 grounding continues