The most important thing to remember when writing narratives for investigations is that they need to be factual. Investigators should focus on who, what, when, where, why and how. Personal opinions and conjectures should be left out of it, because the report will go into many hands and many people will interpret it.
Please note that policies and procedures will vary by precinct and organization, but in general the five rules you need to follow are: 1) use first person, 2) use past tense, 3) use active voice, 4) use chronological order, and 5) be clear and concise.
First person narratives use first person personal pronouns like I, me, we, and us. Basically, they put the responsibility on the writer.
Upon arriving at the scene, I spotted broken glass in the parking lot. We examined it closely.
Past tense means that the verbs need to describe something that happened already. You are describing the past, so make sure you keep the tense in the past. For example, in the sentence above, arrived, spotted, and examined are all past tense verbs.
Active voice means you need to describe WHO did WHAT to WHOM or WHAT. Subject-verb-object.
I (subject) spotted (verb) glass (object).
Chronological order means that you write things in sequence, in the order they happened. In this case, the investigator first arrived, then spotted the broken glass, and then examined it.
Finally, it is important to be clear and precise, and concise. Use as few words as possible to say exactly what needs to be said, with no embellishment.