Deeper Than the Dead by Tami Hoag

Long before DNA testing, four students and a young, elementary school teacher make a gruesome discovery. A serial killer is at large within their idyllic town. FBI investigator, Vince Leone, begins a new FBI technique called "profiling", in order to uncover the truth. New victims are found and authorities investigate the same group of suspects, unaware that someone dangerously close to them is a ***ous psychopath.