One problem with this logline seems to be the comingling of official roles and responsibilities. Detectives investigate, collect evidence, file charges. Prosecutors try cases based on the evidence the detectives present. Prosecutors don't investigate -- it's not their job. They're lawyers; they don't have the experience, training or writ. Just as detectives don't try cases.
Prosecutors may also present evidence to a grand jury. Evidence collected by detectives.
Then there is the job of district attorney (who has prosecutors working for him); he may investigate crimes -- with the assistance of the police or sheriff.
The prosecutor can snoop around on as own, outside his job, of course. But then he's doing so as a private citizen.
So maybe your main character is an "ambitious district attorney".
Call it nit-picking if you want, but the devil is in the details and it's important to get them right in order to establish credibility and prevent readers from getting distracted from the story itself. (Like I obviously have!)
So about the story: what is the plot about? Is it about what he does after discovering these skeletons? OR: does the discovery of the real culprits (his father, the girl friend's mother) constitute the Big Reveal that solves the mystery. If it is the latter then the logline seems to be giving away the ending. Which a logline should not do.
fwiw