I'm reasonably sure this is the same story I saw on AMW a few yrs ago, I tried going to their website but its a huge PITA - man they're just morons.
Anyway, I'm thinking crazed psycho serial killer - that or someone was seriously angry with dad or mom.
A very highly organized psycho did this, was able to leave the car at the border to throw off the searchers and manage to kill all of them, it kind of reminds me of that lowlife in Idaho that broke into a home, killed dad who was on the couch that evening - then got mom to put the two kids into his car with instructions to behave then took her back inside and killed her , killed the boy a little later and they finally caught him with the girl at a Denny's or something.
Everyone thought it was the dads associates because he had a Harley/tattoo's but the killer simply stalked the family because he saw the little girl, saw they had no dog, looked in the window and saw dad sleeping and used a hammer he found in the garage to kill dad.
( yup, the house had no firearms either)
This is a real mystery, anyway, prayers/condolences for the family-what a horrible thing to endure.