Serial Killer Myths Exposed
The killer always leaves evidence at the scene
Don't we wish! There is a theory called Locard's Principle of Exchange which states when a crime is committed the killer will always leave some evidence at the scene and take some evidence away with him. The operative word here is "theory". In theory, this is exactly what happens, at least on the microscopic level. Without this theory, all those crime shows with their fantastic forensics wouldn't be able to go into their second and third seasons. There is always some DNA, some fiber, some spore from a plant, some tire tread, some tool mark...something...to link the murder to the bad guy. When that is accomplished, the happy district attorney goes to court and presents an airtight case and justice triumphs.
Then there is the real world. The world where the fingerprints found belong to everyone but the killer (including the cops, the emergency medical team and the mayor), the DNA is too minimal to test, and when the firefighters came in to put out the fire used to cover up the murder, they drowned every shred of evidence under a foot of water. In reality, those rare few cases with good forensic evidence are the ones that make it to court. Prosecutors hate to lose their cases and ruin their reputations. They make darn sure they have a slam dunk case before they hit the courtroom doors. The rest of the cases aren't prosecuted even though we know who committed the crimes. The killer stays out on the street and his name falls through the cracks and by the time he commits his next crime one county over, the police department in that jurisdiction will have no clue as to who he is.
Bobby Joe Leonard has rarely been outside prison his entire adult life. His felonies cover pretty much every crime. By the time he chooses to commit murder, he has seen enough crime shows in prison to learn about the bad habit of leaving evidence. He brings along a condom with which to commit the rape. He also knows his victim. She is afraid of him and he knows she will not fight him. This means he doesn't have to explain scratches on his face or a bite mark on his arm. After he rapes his victim, he allows her to put her clothes back on. She thinks it is so they can leave but his real reason is to cover up the act of rape. When they find her body, the medical examiner may not be able to prove there was a sexual assault.
Evidence can vary depending on the circumstances, the weather, and how long it has been hanging around. Evidence left out in the woods is likely to be useless. The same goes for bodies that take a year to surface in the water. Other times the evidence is actually collected and stored, but because of human fallibility, "stuff" happens. The evidence locker may have caught on fire or someone simply threw away the evidence because he thought the case was closed. Evidence may become contaminated, as every defense attorney can attest. Evidence also degrades with time so that it becomes impossible to get much information from it.
To have a successful case in court these days, the prosecutor needs DNA linking the suspect to the victim; either his DNA on, in or around the victim, or the victim's DNA somewhere on the killer's clothes or in his domicile or vehicle. Barring that strong evidence, a lot of trace evidence (those fibers, for example) need to be matched in numerous situations where the jury will see it would be impossible to believe the defendant didn't commit the crime. On rare occasions the defendant will be convicted by circumstantial evidence, but usually this is only successful if the jury really dislikes the defendant.
Bobby Joe Leonard strangles Janie with a sleeper hold. He places her in a closet so her body won't be seen immediately upon entering the building. He gets back in his car and goes to work. He is feeling pretty confident that nothing links him to the crime since no one saw him go into the building with her and he didn't leave any forensic evidence at the scene.