In my opinion, I think he "knows" at least one of the victims through FB. Facebook easily allows a person to discreetly keep tabs on a person. Perhaps he wasn't FB friends with the victims themselves, but maybe a relative that often posts pics of them and posted cute family antidotes (a lot of people do that) and based on that, he kept tabs on them and perhaps found out that there was a trip for the bridge planned that day.
Just my thoughts for what it's worth... I'm sure that LE is searching through that, but I doubt that the suspect was FB friends with the girls themselves (however it's possible), but is "friends" with a close friend or relative. He could also be keeping tabs on the case through this same FB relative or friend due to the insider knowledge on the case it would provide for him.
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