I wonder about the hubris of the killer. He has gotten away with something (so far, right?) He may be over confident he will not be caught. I am not sure if this was his first crime, but there must be clues to those who know him of disturbances in his personality.
So two lines of thought.
1) Random killer out on a hunt.
2) Someone who knew the girls and where they would be that day, and had another motive.
So much of understanding the motive is the crime scene, the conditions of the bodies, interviews with witnesses, and any seminal (sorry!) evidence.
With the first, I think a happenstance opportunity for the killer, which will be harder to solve.
If it's the second, good old fashioned police work is needed to investigate the trails in the spiderweb going on.
Hope we get an answer soon and justice for the girls.