The following is theory based on my belief that the murderer was at the Monon High Bridge trail that day hunting, waiting to commit a serious crime. He went there prepared to carry that out on that day.
I've been thinking about the actual day the murders were committed. I think it is a huge factor to be considered in finding a plausible and believable scenario for this terrible crime. And I've felt this from the beginning.
FACT: it was a Monday AND it was school day that was off only because snow days built into the school calendar had not been used up. This was an obscure reason for teenage girls being there hiking on that Monday. The school calendar does show it as a day off if it doesn't need to be used as a make-up day for bad weather. I don't know what the cut-off date is for the school corp to announce that that date is going to be used as a free day.
So how does the fact of this particular Monday (a "non-scheduled" free school day) figure as a clue to who the murderer is? And I'm analyzing this mostly in relation to the numerous posts I've read that suggest that BG is a trucker, one who doesn't live in Indiana at all but passes through often so is somewhat familiar with Delphi. I don't see the likelihood that this type of suspect would have knowledge of the fact that school was out that day. Therefore, I don't see him as a likely suspect if he was there to commit a crime involving teen girls.
Now, let's go with a theory that a trucker who passes through often was planning a murder of a random female, any female, not necessarily a teen-aged female. He is somehow familiar with the Monon High Bridge and remote hiking (which I have difficulty believing, but possible) and thinks it provides good hunting grounds for him. He chooses that day, waits around for a possible victim. And who shows up but Libby and Abby and he decides to target them.
IMO the theory that an out-of-area trucker is the BG is somewhat valid IF he originally planned on attacking a female on that day, not specifically teenagers. That type of person knowing the school calendar to target teens is too much of a stretch for me.
And actually, this theory of BG targeting any female (not specifically teen girls) can also apply to a suspect who lives in Carroll or the surrounding counties. He knows the trail, picks Monday for whatever reason, and goes hunting for a random female, sees Libby and Abby and attacks them.
All that being said, IMO I do believe BG was specifically hunting female teens on that Monday And in order to be there doing that, it's far more likely that BG is someone who lives in the area, knows the Monon High Bridge area very, very well, has familiarity with the school calendar to know that kids would be out of school, and figured there might be some teen girls hiking there that day.
All of this is IMHO.