An interesting article came out this week on Jobsinformed titled "Seven Steps to Successful Workplace Investigations" by employment attorney Patricia C. Perez, SPHR, founder of Puente Consulting.  

In her Step 3, "Gathering Documentation", Ms. Perez summarizes the relevance of the investigator viewing the personnel files of the complainant and the accused.  She writes:

You should automatically pull personnel files for both the complainant and the accused.  The complainant’s file will tell you if he or she has ever filed other grievances, and what, if any, action was taken.  It will also show if the person has a history of complaining about work-related issues or if he or she has an outstanding record but recently received a bad performance evaluation.  This could indicate there is something else at play in the situation.  As for the accused, the file will show if there is any prior history.  It will tell if this is his or her tenth allegation of sexual harassment or it will include accommodations that state how fair he or she is in the workplace.

This is very good advice.   Along these same lines, but slightly different, I suggest in my article "Practice Pointers for Workplace Investigations" (co-written with Los Angeles employment attorney Nancy Bornn and posted on the CAOWI website) that the investigator should interview the complainant before viewing the personnel file so as to avoid any real or perceived bias towards the complainant at the time of the interview.  

Other documents the investigator should obtain, if relevant, are internal policy manuals, employee handbooks, e-mail communications, computer histories, cell phone records, security records, computer log in sheets, sign in books, and any other documents relevant to the facts of your incident(s) under investigation.