I havent commented until now but I believe twhere there is smoke there is fire and in this case it started as a trickle and will end up a raging fire.
This case so reminds me of the Timothy Boczkowski case. He was convicted of the murders of both of his wives by drowing ( one in the bath tub and the other in the hot tub) and recieved life w/o parole in one state and the death penalty in the other.
here is a link to some background info on those murders.. scroll 2/3 rds down the page.
http://www.vanceholmes.com/court/trial_miscellany3.html
In this case I think Petersen is guilty of murdering his former wife and likely his MIl ( who is "missing") as well as the murder of Stacey. I think there is waaay too much here already.. indicating that this is likely a murder not a deliberate running away as he seems to be trying to convince all who will listen. Its highly possible that when they find Stacey , they may find her mother nearby. certainly convient that she was so young when her mom dissapeared.. makes her more dependant upon him emotionally and financially..