Apparently their neighborhood wasn't as secure as initially though. There were a number of sex offenders and pedophiles believed to had been living in that area at the time. Given her notoriety at the time, I'm only guessing someone found an opportunity to harm her and fled. I was suspicious of the parents for a time. But not sure about that anymore.
This is one of those cases where the investigation got so screwed up at the start that barring a complete confession or a scientific evidence breakthrough, I doubt we'll ever know for certain who did it.