The major difference between the NTSB and the military is in the way they do their investigations:
The military looks until they find the base cause... "part NSN XXXX-XXX-XXXX failed, causing the aircraft to depart controlled flight, and condition Z prevented the pilot from regaining control prior to impact".
They issue their "cause of accident" report, and then refer the part to an engineering investigation to determine how it failed, and whether it was a materials flaw, manufacturing defect, fatigue failure, damage from external cause, etc. The other factors are also then referred to their own investigational bodies to determine how they happened, if (and how) they could have been prevented, and how to work around them if they cannot be avoided.
The NTSB will not issue its "cause of accident" report until all of those secondary investigations are complete, and "recommendations and procedures" are written, evaluated, and approved.
They both do the same work, they just issue their "cause of accident" report at different points in the process.