I haven't been reading for a few days, but I have been thinking about the case. I think I may have come up with a possibility for a suspect (not a specific person, but a general category) that I haven't seen mentioned yet, although it may have been in the few posts I've missed. It's also possible that it's a way that this case might connect to the Iowa murders (and many others.)
I may have misunderstood what I've read, but I believe the Carroll County fairgrounds are fairly close to the bridge/trail. Although many carnival workers may be fine upstanding citizens, the lifestyle does a lot of rather unsavory characters. The county fair in Black Hawk County (Waterloo Iowa) appears to be held in July, which means it would have been there at, or near, the time the girls were taken. The one in Delphi/Flora wasn't happening at the time of the girls' deaths, but it could account for someone being familiar with the area but not familiar to the residents.
I've had friends over the years who worked in carnivals when younger, and it is very common for workers to seek out areas to drink/get high, meet up with locals, or even just find some peace and quiet. If a "carnie" had gone to the bridge for one or more of those reasons, he might get very familiar with it over the years, and the place would probably be somewhat isolated if there is a big fair going on close by so he wouldn't be seen much by locals. I think it might be interesting to talk to the owners of Wilson Family carnivals to see if there may have been someone who suddenly quit before they went to Carroll County this year and possibly looked fairly similar to the sketch.
I haven't found the schedule for the carnival company yet to see where they have been, but I'm looking to see if there have been unsolved murders/disappearances/assaults near the areas (especially with similar victims). MOO