I don't know either, but she must have either changed or had her clothing removed. LE said not to focus on her clothes or hair as they may have changed. this was after they found the backpack. not sure if the extra clothes were in the backpack or her clothes were found elsewhere though.
Yes, I kinda got the sick feeling that this was a sex crime and her clothing had been removed, then the perp neatly folded them and placed them back in her backpack. I'm not clear what that purpose would be, other than to just show how sick he is. Since the backpack was neatly placed in plain view, he may just have been sick enough to do that.