It's quite possible that 11 yr old Celina Cass, after having visited her bio-father in the hospital the previous week and seeing him in an induced-coma (does an 11 yr old understand that .. did she believe she bio-dad would die soon?), having an ailing bio-mother, an ailing step-father, a step-sister whom was not in the house with her Monday evening ... felt quite "alone" at her computer and decided to take a walk to the river......
......of course the alternative, abduction and murder, is just as possible but I think the former is more likely.
No pleas from the SF and mother .. I suspect a suicide note was left on her computer .. and it was just a matter of finding her / body.
I believe the reward was an incentive to keep people looking .. perhaps find her before she could hurt herself?