The stranger you know / Andrea Kane.

Enhanced descriptions from Syndetics:

It begins with a chilling phone call to Casey Woods. And ends with another girl dead.

College-age girls with long red hair. Brutally murdered, they're posed like victims in a film noir. Each crime scene is eerily similar to the twisted fantasy of a serial offender now serving thirty years to life--a criminal brought to justice with the help of Forensic Instincts.

Call. Kill. Repeat. But the similarities are more than one psychopath's desire to outdo another. As more red-haired victims are added to the body count, it becomes clear that each one has been chosen because of a unique connection to Casey--a connection that grows closer and closer to her.

Now the Forensic Instincts team must race to uncover the identity of a serial killer before his ever-tightening circle of death closes in on Casey as the ultimate target. As the stalker methodically moves in on his prey, his actions make one thing clear: he knows everything about Casey. And Casey realizes that this psychopathic won't stop until he makes sure she's dead.