Well how do we know anything about anything we didn't witness ourselves? There were multiple deputies present, as I recall. The only conflicting account was the girlfriend who came up with multiple accounts.
You are questioning the validity of the investigation, which is fine, but to what end? Do you have information to the contrary? The prosecution took the evidence available based on an investigation to reach their conclusions, so what do you bring to the query?
Maybe you should read this report and it will answer your questions...