I wonder if the chain of events could've begun with an accident at the P&R and the person responsible panicked. Maybe a teenager or someone who knew another DUI charge or probation/parole violation would land them in prison would be motivated to go to such extremes. For someone like that, perhaps with their movement in the community restricted, the culvert could be at the edge of their awareness space. (Someone here linked this
article which includes a succinct explanation of awareness space. You can skip down to almost halfway, to the paragraph beginning "A person's, including a criminal's, awareness space...".)
I'm so sorry and so hope you and your family get answers soon.