The prosecutor's office is an institution conducting and supervising preparatory proceedings. It is the prosecutor supervising preparatory proceedings who orders the Police to perform particular activities. The prosecutor also makes the final decision on the method of ending the preparatory proceedings: he/she may bring an indictment to the court, or issue a decision on discontinuation of the proceedings, as well as on refusal to institute the proceedings. In both last cases, if you do not agree with the decision, you may file a complaint.
Remember that a complaint may be filed only within the time determined by law. The information about time and method of filing a complaint can be found in the issued decision on discontinuation of the proceedings, or on refusal to institute the proceedings.