It significantly comes after the FAA fined Boeing US$2.5 billion in 2021 for deceiving its safety officials in a separate investigation into two fatal crashes involving a different variant of the MAX aircraft.

On Thursday, a strongly-worded letter sent to Boeing told the company to send “any evidence or statements you might care to make concerning this matter within 10 business days”.

“Any discussions or written statements will be given consideration in the final conclusion of our investigation,” it read.

“However, if we do not hear from you within the specified time, our report will be processed without the benefit of your statement.”

The FAA’s new intervention follows air crash investigators trying to determine if the bolts used to secure the door plug that blew out were installed correctly.

United and Alaska Airlines – the two major carriers that operate the aircraft – both reported finding loose bolts. More than 170 MAX 9s operated in US airspace or by US carriers are still grounded for safety inspections.

“Every Boeing 737-9 Max with a plug door will remain grounded until the FAA finds each can safely return to operation,” the FAA said earlier.

“The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service.”

No Australian carriers currently operate the MAX 9. Virgin Australia and Bonza, though, both operate the shorter MAX 8 variant, with a number of MAX 10s on order for Virgin.

None of the MAX 8s are affected by the incident, as the model lacks the emergency exit that is sealed by the door plug.

It’s the latest incident to damage Boeing’s reputation over the MAX-branded aircraft after two fatal crashes in Ethiopia and Indonesia killed 346 people.

It resulted in the US Department of Justice fining Boeing US$2.5 billion and accusing the planemaker of “fraudulent and deceptive conduct”, “concealing material information” and “engaging in an effort to cover up their deception”.

“The tragic crashes of Lion Air Flight 610 and Ethiopian Airlines Flight 302 exposed fraudulent and deceptive conduct by employees of one of the world’s leading commercial airplane manufacturers,” said then-acting assistant attorney David P. Burns in 2021.

“Boeing’s employees chose the path of profit over candor by concealing material information from the FAA concerning the operation of its 737 Max airplane and engaging in an effort to cover up their deception.”

“This resolution holds Boeing accountable for its employees’ criminal misconduct, addresses the financial impact to Boeing’s airline customers, and hopefully provides some measure of compensation to the crash-victims’ families and beneficiaries.”