Wonder how old Dad is -- the "girlfriend" is 17. Peculiar that she refers to her as "my daughter" -- wonder how her actual mother feels about that.
My bad feeling is that Dad and girlfriend (or one or the other) did it. I totally understand the urge to murder anybody who touches your children. I do think it's a peculiar/suspicious thing to announce in a 911 call. Generally speaking, people are panicking and pleading for help FINDING their child, not thumping their chests about retribution, because they're still hoping the kid is sleepwalking or something.
And, sadly, it's still a thousand times more likely for somebody in the child's life to kill them and make up a stranger abduction than for a stranger to come into a home and take a child out of her bed.