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

Although it seems like she's Sara's partner in crime, Jenna didn't look happy to see Sara talking to Emily. What's more, Sara tells Emily that she's trying to protect her. Sara is about to say from whom, but Jenna overhears and purposefully interrupts their conversation. Maybe Jenna realizes that Sara isn't one to be trusted around the girls and has her killed. That would certainly get her to shut up for good.