Similar to CCTV footage, a video indicating the girls identified their attacker and the person was a known local, that would result in LE naming a suspect. Instead the grainy footage and "down the hill" gathering 15000 tips is an indication the net is still spread quite wide.
In that LE have indicated the case to be complex suggests to me the high possibly the murderer was unrecognized. My theory is that it was a diabolical opportunist, a serial killer who is already long gone from Delphi area.
I certainly hope you're right about taking time. Statistics indicate the longer a case remains unsolved, the greater chance nobody will be successfully prosecuted. I only recently noticed it mentioned another alarming trend --
"Today, the national "clearance rate" for murders, which means the case ends in an arrest, is 64.1%. Five decades ago, it was above 90%."
http://www.businessinsider.com/why-do-so-many-murders-go-unsolved-2015-3
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