Mirror Mirror on the Wall Whose the Killer After all?

Chapter four

"You killed her so I kill you"

My jaw drops open in shock. Who could have done this? And why?
It must have been someone close to Carly. But I was Carly's only friend, at least from what I knew.
I'm so horrified I quickly get up from the chair and climb down the ladder.
I sprint home as fast as my legs can take me. I open the door as softly as I can, as to not attract more attention then I already have.
My mom wasn't home, like she said, and I decide to call Carly's mom. Maybe she will be able to tell me if Carly knew another person she cared about.
With that I find Carly's home number. After 7 rings someone picks up.
"Hello?" Says the person on the other line.
"Hi, this is Rachel, is Mrs. Morris there?" I ask.
"Yes, this is she. How may I help you?" The lady on the other line asks.
"Hello Mrs. Morris, I was hoping you could help me with something."
"Yes?" She asks.
"Umm do you know if Carly had any other close friends besides myself?" I ask hoping she can help.
"Not that I know of. You two were inseparable." I can hear the sadness in her voice as well as the happiness she feels. "If she did, they never came to our home and I never met them." She says.
"Oh well thank you for your time." I say hoping I don't sound disappointed.
"Your welcome."
I start to say goodbye when Mrs. Morris adds. "Just so you know, if you ever want to talk about Carly with someone, I am always here for you."
"Thank you Mrs. Morris, I will." I say, glad that I'm not the only one that doesn't want to forget Carly.

I tell her goodbye and head to my room. I go over to my computer to see if I have any new mail.
I'm expecting a police report or something but I get something totally different.
It's an email from a anonymous address, with the subject "Stop or you die"
I open the email and read through it:

Did you not believe my last message?
Stop looking. She's dead and you killed her. Your dead to me so why not kill you. If you don't stop, you die. And as you will learn, I don't lie...
Best wishes.....Killer

Who is this person? And how does Carly know them? And why?