A few thoughts from a veteran crime reporter:
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2. Focusing on details regarding the bridge or the camera angle are useless in that they do nothing to further identify and capture the suspect(s). Examine the suspect. What can we tell? Well, it appears the blue coat is a buttoned model, as you can see the two sides pulling apart near his stomach. This suggests he's a bigger gentleman. See the hard object in his front right jacket pocket? Yet his hand in his pants pocket. Odd, don't you think? He appears to have a mustache, straight brown hair.
3. Police are sitting on a lot more. Given that they have video, most likely, releasing even a two-second clip of him walking might shed light on his posture, gait, etc. Not to mention higher photo quality. I have a suspicion that the entire encounter was captured, but the contents of that video/audio are either too sensitive to release (e.g. cries for help) or could jeopardize the investigation (e.g. the perpetrator telling the victims he's going to shoot them).
4. If the phone wasn't recovered at the scene, they have already tracked its movement, but that's not entirely helpful as it could've been discarded anywhere or turned off and the SIM card removed. If it was recovered, and I believe it was, they've got a digital record of everything the girl did prior to this encounter. That usually indicates he is a stranger.
5. His knowledge of the area would indicate he's familiar with the terrain. Who just happens to stop by a random trail near Delphi, Indiana for a walk? Had the perpetrator intended to kill at that day and that particular time, there are far greater odds of finding a victim at a location other than where this occurred. This suggests to me the attack was random and not planned.