>>> Inquiries won't help if they stop at Boeing, or contractors Boeing used.

The investigation hasn't stopped at Boeing nor Boeing contractors.

It's not covered in the NYT article linked in this HN topic, but the Wall Street Journal article that was first to report the Justice Department investigation says the NTSB is "seeking to interview Federal Aviation Administration Officials in the Seattle area who oversee Boeing's manufacturing".[0]

Sounds like an initial step towards identifying any fault that lies with the FAA and FAA oversight.

[0] https://www.wsj.com/business/airlines/justice-department-ope...