Part 1 - Case study. Taken from a TV show Dead of Night episode, "Blood Never Lies".
Summer 2003 - Las Cruces, New Mexico - State University
First break - A couple is driving through the desert a couple miles from the town. The female passenger notices something odd and asks her partner to stop the car. He gets out and walks closer and sees the dead body of a female laying face down wearing only a top. Shaken, he returns to the vehicle and they call 911. Detectives arrive on the scene within an hour.
Second break - After processing the scene, covering the victim's hands with paper bags to preserve DNA that might be under the fingernails, taking castings of what appears to be the vehicle's tire tracks that dumped the body in the desert, and noting the perp tried to burn the body as evidenced by scorch marks on her back, they determine it looks as if it is a sexual assault crime. Not knowing the victim's identity yet, they find out a missing person report was filed that same morning (second break). The victim is 22 year old Katie Sepich, a graduate student at NM State, a lovely young woman who had the same boyfriend who she met and dated as an undergrad. After graduation he moved back home to help with his family business while she stayed on for grad school. They were still together and earlier he had bought her a diamond ring (it must have been a promise ring because they don't say they were engaged).
Within the first day of this case, LE has a missing person report, finds a body through curious civilians and discovers there's a match. LE can determine that point C (dump site) isn't the actual crime scene (which will be point B).
Why am I doing this? To show the disadvantage LE has when it comes to solving a case when there's no body. Even though I haven't touched on the investigation yet, imagine if the civilians hadn't noticed a body in the desert on the first day Katie was missing. If the body had been found later (not sure how many days - one or two months, less?), the DNA would be compromised or even nonexistent. Third break - LE was able to extract DNA from under her fingernails showing she had put up a fight. They also were able to collect evidence of the sexual assault which matched the DNA from under the fingernails. Now they know who they are looking for and can get a ethnicity profile of the person which narrows it down a little bit.
At this point in the investigation, in the order statistics usually suggest (I'm beginning to wonder about their accuracy these days), the significant other is first on LE's list when determining a suspect. You'll see why, especially in this case, this can be problematic if you're the significant other of someone who is found murdered.