Agree that the simplest explanation is often the real answer. Unfortunately, pedophiles kidnapping and killing young girls, while rare, is a simple answer, too.
Here's a link to Mark Klaas's web site with some statistics about child kidnapping. It's very good reading for those who want to learn more about this sad subject:
http://www.pollyklaas.org/about/national-child-kidnapping.html
It's been reported by a news photographer that the girls clothing was seen in the creek the day they were discovered, so sadly it's the most likely scenario.
Here's an update to that report written in 2011. Not much has changed.
https://www.ojjdp.gov/pubs/249249.pdf