In a Police Procedural, the writer must adhere to the basic elements of mysteries while at the same time incorporating what makes the story a police procedural. Sometimes it can be hard to distinguish a thriller from a police procedural novel since thrillers also may be filled with lots of detail about the crime(s) and how solved. Because it must be solved. That is part of the covenant between writer and reader.
Additionally, in order to be classed as a police procedural mystery:
1) The investigator is a professional in the public safety world. The investigator might be a police officer, detective, EMS person, firefighter, and so on.
2) The emphasis is on factual operations of police investigations rather than on character development. Characters tend to be tropes, but investigative rules must be followed.
3) The perpetrator may or may not be known from the beginning.
4) There can be one crime or more than one. If more, the crimes may seem unrelated but connect somehow. Or the novel may feature several unrelated crimes being processed by the precinct or department and have multiple investigators, a sort of ensemble cast.
5) Solving the crime is a group effort. Police procedurals demonstrate a range of police techniques including forensic, interrogation techniques, arrest and search warrants, interviews, autopsies, and behavioral science protocols.
6) Police procedurals provide details of the crime scene investigation, from gathering evidence to processing it. A realistic portrayal is demanded. A team is needed for different aspects of the investigation.
7) There is typically a graphic description of the crime scene. Violence often takes place “on stage” with varying degrees of explicit violence or gore described. Sometimes it is provided through the investigator’s reconstruction of the crime.
8) The POV can be entirely or mainly from the professional investigator’s stance. Or the villain’s POV may alternate to show his/her thinking, particularly if a crime spree is depicted.
9) The crime’s perpetrator in police procedurals, though not always in real life, is a worthy intellectual foe for the investigator.
10) Extremely intensive research to write police procedurals includes spending time with safety professionals as they work, if at all possible. If not, spend other time with them to get the small details right.
If you want to read more about types of mysteries and how to write them, check out my June-December mystery posts on the second Tuesdays at Writeon Sisters and search this "Parsley, Sage, and Rosemary Time" blog for earlier posts.
Before writing a police procedural mystery, read these authors. Of course, there are many other great ones, too!