Frailty

Fenton Meiks visits FBI Agent Wesley Doyle claiming that his brother Adam is the "God's Hand" serial killer Doyle has been hunting. Meiks says Adam has committed suicide, prompting Fenton to fulfill a promise to bury Adam in a public rose garden in their hometown of Thurman. He begins to tell Doyle about the boys' childhood and suggests that the bodies of the God's Hand victims are buried in that rose garden. Meiks continues telling Doyle his story as the two drive to Thurman.
When the brothers were children, their father told them that he'd been visited by an angel and tasked by God with destroying demons disguised as human beings. As punishment for his initial disbelief, he demands Fenton dig a root cellar. He explains that this mission is now the responsibility of the three of them and must be kept secret from all others. The father's modus operandi is to wait for the angel to give him a list of names of those who must be destroyed. He then abducts an individual from the list, takes them to the family home and, with his sons present, touches them, which, he says, grants him a vision of the crimes the demon has committed. He then finishes the victim with an ax and buries the body in the rose garden.
Adam believes in their father's mission and says that he sees the same visions of the demons' crimes that their father sees. Although he initially goes along out of fear, Fenton doesn't believe and is convinced that their father is psychotic and has brainwashed Adam. Eventually, Fenton tries to stop the crimes by telling the local sheriff what has happened. The sheriff visits, but is about to leave when Fenton insists that he search their root cellar. Horrified, the father kills the sheriff.
He blames Fenton for revealing their mission and thus forcing him to kill the sheriff. He further implies that the angel has told him that Fenton is a demon too, but instead of killing Fenton, he locks Fenton in the root cellar hoping Fenton will have a divine revelation. After countless days of starvation, Fenton tells his father that he has indeed seen God and is ready to take his place in the mission.
Upon the next abduction, Fenton is given the ax to deliver the death blow, but kills his father. As Fenton moves to free the hostage, Adam takes up the ax and finishes the victim. The two boys bury the bodies in the rose garden and Fenton asks Adam to bury him in the rose garden should Adam ever decide to "destroy" Fenton.
Puzzled by his phrasing, Meiks reveals to Doyle that he is Adam and that he has destroyed Fenton, who grew up to become the God's Hand serial killer. Further flashbacks reveal that those his father abducted had indeed committed horrific crimes and that Adam had shared in his father's visions of those crimes, just as he'd always claimed. When he touches Doyle, a new vision reveals that Doyle had murdered his own mother. Meiks tells a dazed Doyle that Doyle's name was given to him on the list of demons to be destroyed. Doyle protests that people at the office saw Adam. Adam declares that God will protect him and then strikes down Doyle with the ax.
Those who saw Meiks at the FBI office, including Agent Griffin Hull (who can only recall speaking with him face-to-face and shaking his hand), inexplicably remember nothing about "Fenton" Meiks' appearance. Surveillance videotapes all have a distortion when played that obscures Meiks' face. The investigation then proceeds as Adam had predicted.
Agent Hull visits Enid County Sheriff Adam Meiks to deliver the news about Fenton, but doesn't recognize him. Shaking Hull's hand before he takes his leave, Adam tells him he's "a good man," indicating he's seen that Hull has not committed any crimes. He departs, Adam telling his pregnant wife that "God's will has been served."

Question: Who does Wesley Doyle work for?
Answer: FBI