The police procedural, or police crime drama, is a subgenre of detective fiction that attempts to convincingly depict the activities of a police force as they investigate crimes. Although traditional detective novels usually concentrate on a single crime, police procedurals frequently depict investigations into several unrelated crimes in a single story. Traditional mysteries usually adhere to the convention of having the criminal's identity concealed until the climax (the so-called whodunit), whereas in police procedurals, the perpetrator's identity is often known to the audience from the outset (the inverted detective story). Police procedurals depict a number of police-related topics such as forensics, autopsies, the gathering of evidence, the use of search warrants, and interrogation.