In some crimes there are no witnesses and in the absence of self-incriminating statements by a suspect, the only means of obtaining a conviction may be through physical evidence (evidence with viable DNA, a blood sample, a fingerprint, etc.). In any crime, sharing knowledge of physical evidence with suspects may loosen tongues and stimulate confessions. DNA, fingerprints, or serologic evidence are tough to debate and bring many criminals to a place where their lips move easily.
Note re: private investigators sharing knowledge of physical evidence with a suspect: Interestingly enough, private investigators work under a burden created by ethical constraints that police detectives do not labor under. While courts have consistently held that police may lie to a suspect to stimulate a confession without tainting that confession (we recently watched this in an episode of The Closer), very few private investigators can credibly present statements obtained by deceptive means. By “very few” we mean in the few instances where the PI has investigated an individual who’s extremely unsavory or has committed a particularly heinous act, then jurors are more likely to trust the PI’s statements even if the PI lied to obtain them. This is great fodder for a story.
In a crime scene, the area searched and the evidence sought will depend on the crime under investigation. In crimes of violence, the crime scene tells the detective what happened but the detective has to be able to read the signs left by the evidence (signatures of crime include fingerprints, blood stains, bullets, bullet holes, tool marks, fibers, hairs, glass fragments, fingernail scrapings, DNA samples, as well as items added, overturned, removed, or displaced).
Keep in mind that the suspect is also part of the crime scene. What does he leave at the crime scene and what does he take away from the scene? Such evidence helps to prove that he was there. If the police take him back to the crime scene after his arrest, the evidence of his presence at the scene, when presented in testimony in the courtroom, may serve only to prove that the police took him there. This may cause your fictional PI to think twice before taking a possible suspect to a crime scene.
It’s important to make the distinction between what crime scene investigators for the police consider a crime scene and what the rest of us, including PIs, consider a crime scene. In the latter instance, a crime scene is really just the place where a crime happened, which has returned to everyday use. On the other hand, what police and crime scene investigators consider a crime scene is that area where, such as the space inside the yellow tape, careful protocols for evidence recordation and extraction are followed.
Have a great week, Writing PIs