First, has this been submitted to Cold Justice for consideration? in the last episode I saw they mentioned having to interview 90 witnesses and I think this case could benefit from someone willing to speak to all Billy's neighbors, classmates, etc. Does the request need to come from LE?
While I wouldn't rule out a cover up by LE, I am not sure that is what happened here. I think that LE got an idea in their head and that clouded their judgment and how they moved ahead in the case. Billy's story was suspect and pieces didn't add up. I agree with others who believe that this was probably because Billy knew his attackers and/or was victimized in a way he didn't want known. Sadly, LE didn't see it that way and this knowledge clouded their judgment from that point on.
I also think that this was done by older teens/ young adults. Many of the things that made Billy the wonderful person that he was could also make him the target of bullies -- did well in school, in choir, dressed up as Santa for neighborhood kids. They need not be someone directly known to Billy. As a kid, I was the target of bullies and one day walking home from school I became a target of boys I never knew before. To this day, I don't know who they were or how they knew me. They lived in my neighborhood but didn't go to my school but since I never " hung out" in my neighborhood or hung with any boys, I have no idea why they targeted me. I can only assume they knew someone at my school. (I went to an all girls school)
It was very frightening, much more so then the bullies at school, precisely because I had no idea what made me target them. (They harassed me several times until the main antagonizer got so close to me and I felt physically threatened some hit him, something I had never done before. I guess being hit by a nerdy girl was not something that he wanted to experience again so they never bothered me again)
Anyway, my point is, if these bullies were someone with no known connection ( maybe a younger sibling knew him or they saw him do his paper route) to him, that would be terrifying enough, add on the violent strangling and you have enough fear to make Billy susceptible to any threats against himself or family. I think they are young adults due to the fact that he was strangled three times. Either they didn't want to kill him the first two times, just really terrorize him. Or they did want him dead but because they were young did nt think to do it another way ( e.g. knife) that would be bloody. Alternate theory is the budding psychopath who wants to experience a death this way.