If you go to "The Charlie Project" website, and click on "South Carolina" you will see that there are a staggering number of missing persons in South Carolina. Also from the news you can see a very high number of killings that have occured in recent years, a number of them young women.
Shannon McConaughey was killed by somebody. Normally there would be an investigation by somebody who has that job, and the evidence would be lain out and a person could look at the evidence and try to guess who did it.
Unfortunately, in the Shannon McConaughey case, the police decided who they wanted to charge with the crime, before they did the investigation. They were investigating not with the intention of "finding" the killer but of proving they had arrested the right person.
So, while they were investigating that colored their actions.
When they finally finished investigating they gathered up all of the evidence they had collected against those individuals. There wasn't any.
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As for "One co-defendant told investigators ... raped and shot McConaughey.", it's very easy to get somebody to say what you want them to say when you have physical control of them and their environment. In some places the police have refined this to an evil art. They put a person in a small cage for weeks or months or longer and carefully cultivate the story they want, using the complete physical control of the environment they have.
Any good historian will tell you that there have been many countries in recent history that have brought the art of getting untrue facts from prisoners to perfection. Even generations ago there were experts who could get anybody to say literally anything. Today unhealthy individuals in governments have studied that more precisely and it can almost always be done without physically harming the person, simply by using psychological pressures. You can watch videos on YouTube of false confessions to get an example.
One case that I noticed recently in a documentary was interesting. A young Swedish guy, Malthe Johansen, was accused of a sex crime at a school. The police did not want to spend weeks and weeks investigating, so they decided to shortcut things. He had been raised by two women, so the police sent two female police officers to talk to him, thinking he would be more likely to cooperate with them. They told him that they had video proof that he had commited the crime. He said that he did not remember it, but if they had video proof then he must have done it as a split personality or something. The two women then asked him if he would be willing to sign a confession, even though he didn't remember commiting the crime. Since it was two women, he said "of course" and signed the confession. Eventually he was exonerated, but he died shortly after. In his country in Scandanavia there appear to be a number of videos and articles about his case that you can look at if you speak Viking, or you can run them through Google Translate.
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As for "There is clearly a correlation to these disappearances." Yes, there clearly is. This large number of murders and kidnappings etc did not spring up out of the ground, they were caused by something. You would ask a sociologist who has studied that kind of thing to speculate about that.
When there is an individual murder, or group of murders commited by one person, and you want to see the 'cause' or what connects them, you would ask a psychologist to speculate about that.
You mention Crystal Soles too. You might go to the "Charlie Project" website and look at the page on South Carolina and gather all the cases that might seem to have been from a single cause, in this case young women who were probably attacked for a reason involving the fact that they were young women. You may find more and more cases that seem to be very closely related, and if they are solved the killers will have certain things in common.