The U.S. Federal Aviation Administration (FAA) will keep all Boeing 737 MAX 9 aircraft grounded for the safety of American travelers until extensive inspection and maintenance are conducted and data from inspections is reviewed, it said in a statement posted on its website on Friday.

After reviewing Boeing's proposed inspection and maintenance instructions, the FAA determined it needed additional data and is requiring Boeing to provide it.

Accordingly, the FAA is requiring plug-door inspections of 40 aircraft.

If Boeing's inspection and maintenance instructions are approved, operators will be required to perform that regimen on every aircraft before it is returned to service, the FAA said.

The FAA decisively grounded approximately 171 Boeing 737 MAX 9 planes on January 6 after Alaska Airlines Flight 1282, a 737 MAX 9, lost a mid-cabin exit door plug while in flight a day earlier.

The agency has launched an investigation to determine if Boeing failed to ensure that completed products conformed to its approved design and were in a condition for safe operation in compliance with FAA regulations.

The FAA said it will continue to support the National Transportation Safety Board's investigation into Alaska Airlines Flight 1282, which is independent of its own investigation.