Sigh.....
To repeat:
This isn't just a case of some random diary being stolen. This involves the diary of the daughter of one of the major parties' Presidential candidate being stolen and the contents of that diary being posted online just prior to the Presidential election. That obviously (at least to some of us) brings up a whole host of possibilities that go far beyond mere petty theft of a woman's book.
Was it an attempt to influence the election? That needs to be investigated, and by someone other than the local police.
If it was an attempt to influence the election, who did it? Since it would have clearly been an attempt to help Trump win, and given that we already know the Russians worked to help Trump in the 2016 election, they would be an obvious suspect.
And that leads to a much bigger possibility....should this be a counter-intelligence operation?
Given the specific circumstances here, at the very least those need to be investigated, and the FBI is the appropriate agency to do so.
And to repeat again......duh.