That Darkness by Lisa Black

As a forensic investigator for the Cleveland Police Department, Maggie Gardiner has seen her share of Jane Does. The latest is an unidentified female in her early teens, discovered in a local cemetery. More shocking than the girl's injuries--for Maggie at least--is the fact that no one has reported her missing. She and the detectives assigned to the case (including her cop ex-husband) are determined to follow every lead, run down every scrap of evidence. But the monster they seek is watching every move, closer to them than they could possibly imagine.

Jack Renner is a killer. He doesn't murder because he enjoys it, or because he believes himself omnipotent, or for any reason other than to make the world a safer place. When he follows the trail of this Jane Doe to a locked room in a small apartment where eighteen teenaged girls are anything but safe, he knows something must be done. But his pursuit of their captor takes an unexpected turn.