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You can write the story any way you like, but you run the risk of breaking your reader's attention to your tale, by suddenly making changes to your method of telling. Instead about thinking about ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/31061 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
You can write the story any way you like, but you run the risk of breaking your reader's attention to your tale, by suddenly making changes to your method of telling. Instead about thinking about the welfare of your characters and the progress of your plot, they might start thinking, "Why the change in POV?". To minimize that distraction in this case, why not have your third party narrator announce that the following comes from the filed report of the young detective; before reading the report (in first person) to the reader. This unifies the whole book under a single point of view, while still allowing you to control information and insight by using first person for the first part.