All Shall Mourn by Ellie Marney

The third and final book in the None Shall Sleep Sequence.
Simon Gutmunsson is on the loose… Since the disastrous events of the College Killer case, the FBI is coming to terms with the fact that while catching one sociopath, they’ve released another. Chillingly manipulative, frighteningly intelligent, and wholly insane, Simon Gutmunsson is the worst of the worst. Now he’s free, and on the run with his twin sister Kristin, and nobody in the FBI is smart enough to keep up…

Travis Bell’s last investigation for FBI Behavioral Science nearly killed him. Supported by family and his partner Emma Lewis, now Travis just needs to get back on his feet. But Emma has been recruited to help the FBI hunt the ultimate sociopath, Simon Gutmunsson, and there’s no way Travis is letting her go into this battle alone…

Emma Lewis has gained some perspective on life, and as her emotional walls come down, she realizes that only by working as a perfect team will she and Travis have any chance of beating the Gutmunsson twins at their own terrifying game. So in a journey that will take them from Moroccan souks to Mexican Day of the Dead celebrations, the stage is set for Simon and Kristin and Emma and Travis to meet up in an epic final showdown.