>> then one ended up missing. >> nobody's heard from her. nobody knows where she is. >> something's wroong. something's bad wrong. >> where was skylar. someone was hiding something. >> it was the fbi. they were serious. >> twitter, instagram, facebook. you just see things. it starts to get ridiculously out of control. . >> mean girls. mean enough to kill. >> tonight we take you into a secret world that's going to stun you, teenagers, social media and murder. >> she was like, well, she'll blackmail me. . >> people come if all shapes and sizes. >> a clique of mean girls. were they mean enough to kill? >> it was such a shocking and scandalous kind of case. >> this was diabolical. >> evil comes in all shapes and sizes. . >> hello, welcome to "dateline." . for sky lar's parents, she was all they could have hoped for. a straight-a student and then their world turned upsidedown when their only child seemed to