20/20: Three 16 Year Old Best Friends, Two Of Whom Decide To Kill The Third

Best friends and high school sophomores Skylar Neese, Shelia Eddy and Rachel Shoaf were inseparable.

Living in Morgantown, West Virginia, the then-16-year-olds were pretty and sociable, taking selfies and spending time together. Shown here in this Facebook photo together is Neese on the right, Eddy on the left, and Shoaf in the middle.

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But everything changed on July 6, 2012, when Neese’s parents discovered she was missing. Six months later, Shoaf told authorities that she and Eddy had stabbed their best friend Neese to death.

Watch the full story on ABC News' "20/20" TONIGHT at 10 P.M. ET