|“||Tracy... Let's just go home. I was only playing. Why'd you have to be such a baby? Tracy...||”|
— Jeffrey hunting Tracy Belle
A resident of Ozona, Texas and the son of a school guidance counselor named James, Jeffrey was born with a severe allergy to dairy, necessitating every food product in his home containing even small traces of it to be sealed shut with duct tape. After his mother abandoned him and his father on April 2006, Jeffrey came to deeply resent other children, who his father would spend almost all his time with due to his job. This resentment ultimately caused Jeffrey to bludgeon a schoolmate, Robbie Davis, to death with an aluminum baseball bat after luring him deep into the woods on August 28, taking his backpack afterwards as a trophy and keeping it in his dresser drawer at home. While he was out hiking one day after killing Robbie, Jeffrey stumbled across the body of a local hermit named Joseph Finnegan, who had died of a heart attack while setting a bear trap. Hiding Finnegan's body by covering it with leaves, Jeffrey broke into the old man's house, used it as a hideout, and made it appear as if Finnegan was still living there by taking in his daily deliveries, including dinners dropped off by his father.
In the span of a day, Jeffrey claims two more victims, Nicholas Faye and Sarah Peterson, killing them in the same manner as Robbie and then taking their red baseball cap and schoolbag, respectively, afterwards as trophies, hiding them in Finnegan's house. When the BAU arrives in town to investigate the case, Jeffrey attends an assembly with the other neighborhood children and is told about the unsub on the loose (who was initially believed to be an adult) and is instructed to use the buddy system for safety. During the course of the investigation, the BAU suspect Finnegan of being the unsub. Jeffrey is later taken along to the local police station when his father, who is helping with the investigation, attends a meeting discussing a profile created by the BAU for the unsub. James eventually discovers that his son is the unsub when he finds Nicholas's hat, prompting him to call the school and lie about Jeffrey being sick in an effort to protect him. Shortly afterwards, James is arrested as a suspect due to the BAU finding his fingerprints on the discarded dinner trays in Finnegan's house and Nicholas's hat on his person.
With his father in custody, Jeffrey approaches Tracy Belle at a bus stop after school is out, offering to walk her home (through a shortcut) when the boy she was supposed to be buddies with leaves her alone. While leading Tracy to the woods Jeffrey becomes irritated when she walks too slow and begins complaining, prompting him to throw her school bag away, shove her to the ground, and take a swing at her with his bat. Jeffrey misses and is kicked in the leg by Tracy, who runs off into the woods. Giving chase, Jeffrey tries and fails to lure Tracy out by claiming that he was just playing around and can take her home if she comes out. Jeffrey eventually finds Tracy hiding behind a tree and attacks her, swinging wildly his bat as she desperately crawls away, avoiding his blows. Before he can kill her, Tracy is saved when Gideon, who deduced that he was the unsub after interrogating his father, grabs Jeffrey and pulls the bat from his grasp. Jeffrey is last seen being taken away in the back of a police cruiser with Gideon, who asks him why he hurt all those kids. In tears, Jeffrey responds by saying "Because I wanted to" before placing his hand on the car window after his father does so on the outside.
The unsub was initially profiled as an adult, who would be easily trusted by children, shy, physically fit with a kind disposition, and small in size. He is familiar with the area of Ozona and has probably lived there for his whole life. Due to the acceleration of his cooling-off period, he could strike at any time, with his confidence building with every kill. Even though the unsub is referred to as a "he", women cannot be ruled out.
It is later clarified that the anger caused by the combination of his mother leaving him and his father never being around caused Jeffrey to develop an intense hatred of other children, who he felt were stealing all his father's attention away from him. He killed out of rage but is also methodical and organized. After claiming his first two victims, Jeffrey becomes more brazen and violent, continuing to bludgeon Sarah Peterson even after she had died.
Jeffrey targeted children who went to the same elementary school as him. He would lure them deep into the woods to ensure there would be no witnesses or anybody who would hear them scream as they ran from him. He would then chase after them and bludgeon them to death with an aluminum baseball bat that he hid under a pile of leaves against a tree. Afterwards, he would take their backpacks as trophies, leaving them at Joseph Finnegan's home, but in the case of his second victim, Nicholas Faye, he took his baseball cap as a trophy and kept it in his home. As his rage begins to escalate with each murder, he would bludgeon his victims even after they died.
Jeffrey committing his murders in his adolescence and his habit of luring other children to remote areas where he would beat them is reminiscent of Jesse Pomeroy, who did the same thing, though his victims in those crimes survived. Both also had birth conditions that made interaction with other children difficult (Jeffrey had a severe allergy to dairy, while Pomeroy suffered from epilepsy and a pale eye), were abandoned by one parent and cared by the other alone, committed their first crimes when they were twelve years old, and visited a police station while their crimes were being investigated.
- August 28: Robbie Davis
- October 24: Nicholas Faye
- October 25: Sarah Peterson
- October 26: Tracy Belle (attempted; swung at repeatedly, but missed)
- Cameron Monaghan, the actor who plays Jeffrey, would go on to play Jerome Valeska, a psychopathic serial killer, on the crime drama Gotham.
- At the age of twelve, Jeffrey is the youngest serial killer to have ever appeared on the show, and also the third-youngest killer in the show's history, the first two being Danny Murphy and the sons of the Romani Family, who were nine and ten years old at the time of their crimes, respectively.