Support your belief for the more important of the two reports: the initial investigation report or the follow-up reports.
My belief is that the more important of the two reports is the initial investigation report. Written clearly, this report establishes the who, what, when, where, and why to the best of the investigator's ability. If those questions are not investigated when memories are the freshest they will ever be, when physical evidence can be gathered and logged in, when witnesses can be identified even if they are not yet interviewed, the follow-up reports may not be able to reference the person who actually saw the accident or the bombing. Cameras which recorded the incident and gathered for the initial report may have been recorded over if not immediately found and gathered into evidence. The follow-up reports are also important to make sure that each lead is followed, but the initial thorough investigation report is vital to getting all the important information.