Because none of the kids' movements were predictable on graduation night, that seems a better argument for a random crime of opportunity. The girls, for example, coming to someone's attention, or someone noting Stacy's car at Delmar. I posted about a home invasion by teenagers the other day in which local, related people were originally suspected. There's a bias in law enforcement (and human nature) for things that can be explained within the universe of knowns. That's why they start investigations by looking at intimates, family members, friends, colleagues, etc. It's worth noting that focus shifted to guys like Cox and Garrison, who might have had no real relation to any of the women.