Nicholas Ivie, 30, of Provo, Utah, was killed Oct. 2 while he and other agents were investigating a sensor that had become activated near Naco, Ariz., a few miles from the Mexican border.
The Pima County Medical Examiner released the autopsy report Wednesday, CNN reported.
Ivie was killed by a "penetrating gunshot wound of the head involving the brain," the report said. A second agent was wounded in the shootout.
Preliminary findings by the FBI suggest Ivie was shot when agents began firing in the mistaken belief they were under attack. An investigator said soon after the shooting that the only shell casings in the area appeared to have come from the agents' weapons.