There's Another Dead Body on Pretty Little Liars, but Who Is the Killer?

Mary still claims to be the victim of Archer's scamming and her sister, Jessica DiLaurentis. But we have a feeling that she isn't all that innocent. Mary is in charge of Alison and is able to watch her like a hawk. It definitely looks like she is abusing that power. We also can't forget that she was working with Archer! It's very possible that she knows that covering up what happened to Archer is in her best interest. Could it be she's killing off everyone involved in Jenna's group? Could Noel and Jenna be next?