I'm sorry, but there is no right to have someone prosecuted. Criminal prosecution is not "victim vs. criminal." It is society that is harmed and the state represents the population as a whole when they proceed with such.
If your argument is that you suffered some injury because of the FBIs failure to investigate, that claim will go nowhere. Your first step (you apparently have heard of the FTCA) is to file a claim with the agency. You don't need to name anybody, just the name of the agency you allege caused the damage. You can use the SF95 form for this.
Note that there's a two year limitation period on making such claims.

