Distill and report first the essence of your discovery and your recommendation. Offer a brief recap of what happened, with what results, and why. If cause cannot be determined, say so.
Then mention recommendations—or possibly action already taken—to prevent a recurrence.
Follow with details about your actual investigation procedures and specific findings.
Specify if this is a one-time, infrequent, or recurrent incident. Such information would suggest stronger-than-normal preventive measures, replacement of equipment, or revised procedures.
Include, if significant and possible, time and dollar loss.
Be careful that details are exact and plentiful. Your report may later be the basis of legal action.
Identify subjective statements as such, distinguishing them from investigative facts.
Don’t shroud your details in the passive voice in an effort to sound “official.” If you are authorized by your company to investigate, your report will be official. Active voice and a few pronouns (“we decided to replace…” vs. “it was decided to replace…”) will breathe life into your writing.