So this is essentially on the FCC for approving an quality assurance scheme that leaves them with no clue to figure out if this event would have been prevented if Boeing had followed it's own processes.

The problem for this investigation is that it's not a single door but an entire line of planes designed, made and inspected using processes and policies that the FCC should probably never have approved in the first place creating an situation where half the us aviation industry would collapse if a halt operations until everything have been reinspected under a more correct regulatory framework.

So there is a huge pressure on the NTSB for finding something fixable wrong as the alternative is to start the certification process for both the MAX 8/9 designs and the Boeing manufacturing lines in question from scratch.