I believe Hackett's middle name is Peter, but his first name is not Peter.
I don't think he is involved. I do think the call made to Shannan's mother was a fabrication to get her mom to stop looking for her.
Think about it. If you killed Shannan would you call her mother and use your real name to get her off your back?
Nah...
I might be forced to if she was the one that got away for a little bit, running around the property calling the cops, and they'll eventually be looking for her in the backyard of my community, etc. Yeah, as much as I don't want to, I might have to make that call (of course, it will be from a throwaway cell) if something else might eventually link me to her.