I think this could go above the line ....?

It seems pretty clear the poor boy committed suicide, otherwise ... he went in for a swim?

The parents are 'unmoved' because they have other things to think about ... and are not unused to tragedy and disaster, presumably. It never occurred to me that there was anything unusual about their reaction; I could imagine the farm people of my family and acquaintance reacting in much the same way.

As for what they threw off the bridge ... now that is the big question - and I fear we'll never know ... !