Poor little girl ... I agree that they should investigate the family further, just in case. But it is plausible that the dog did it accidentally. Freak accidents do happen. My great-grandmother had low blood pressure, and while she was coming back from feeding the chickens, she fainted and fell -- with her neck across the wire fence. Strangled her. Freak accident, no one's fault.
What makes you think they didn't?
"Freak accident, no one's fault."
Well, according to your logic, they should have investigated the family, anyway. They should have brought all of you in and put you under the bright lights until someone confessed.