The Highway by CJ Box

It was Danielle and Gracie's secret. A 1000 mile drive to visit Danielle's boyfriend in Montana. But now the girls have simply vanished and Cassie Dewell's investigation will lead her to the FBI's Highway Serial Killer Task Force, a spate of roadside sexual mutilations and murders and the lair of a predator who styles himself The Lizard King.

There's a killer on the road. A predator who calls himself the Lizard King. And two young women are lost in his hunting grounds... It was Danielle and Gracie's secret. A teenage adventure. A 1,000 mile drive along the spine of the Rocky Mountains to visit Danielle's boyfriend in Montana. Their parents were never to know. But now the girls have simply vanished. The only person who knows they're missing is Danielle's boyfriend. He persuades his father – a disgraced, suspended cop – to search for them. But he too simply disappears...