I don't know about this one, but something stinks.
Scenario 1: Grandma and Grampa have a history of physical abuse and lost any control they held in check up to now.
Scenario 2: The father felt bogged down with a daughter and his girlfriend can't stand kids...ala Susan Smith Syndrome.
The thing here is...do the grandparents know? A parents' unconditional love for their son...or he pays the rent and they'd be homeless without him.
He's a suspect in my mind because if it were my daughter I be so high on MD prescripted downers from grief and despair and revenge that I barely be able to speak my name.
Then again, Polly (can't remember her last name) was indeed grabbed in Petaluma, Ca..
So what the hell do I know?