We Are All Guilty Here (North Falls #1) by Karin Slaughter-Used

Welcome to North Falls—a small town where everyone knows everyone. Or so they think.

Until the night of the fireworks. When two teenage girls vanish, and the town ignites.

For Officer Emmy Clifton, it’s personal. She turned away when her best friend's daughter needed help—and now she must bring her home.

But as Emmy combs through the puzzle the girls left behind, she realizes she never really knew them. Nobody did.

Every teenage girl has secrets. But who would kill for them? And what else is the town hiding?