My first post on the site. Thoughts:-
Logical assumption they where watched/followed and stalked over the bridge.
1. Why force them down the hill in the direction of the creek. Was he herding them towards a vehicle or felt safe enough to attack them near the creek? Could the area be easily seen from the bridge and other areas including the houses nearby? How often do Locals walk the trails there? Was there any areas in that direction a vehicle could be parked?
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I have this theory he was already under the bridge. I had this theory before the DN living under bridges statements, to clarify.
Whether he was under the bridge or not, he may have wanted to get them under the bridge, thinking it would be private. Because I believe this was a crime of opportunity, and not a calculated hit, the crossing of the Deer Creek, IMO, came by surprise because one or both girls ran, or he heard something that convinced him he needed to move them.