Sheriff's department officials released the information during Thursday's news conference after receiving preliminary autopsy results.
Authorities said that Marnie Warren, 19, had been shot four times in the head and her half-brother, 13-year-old Bradley Kyle Warren, had been shot 11 times in the head, face, chest, arm and back. Marnie was found in the middle bedroom of the mobile home, and Kyle was found in the back bedroom.
Another sibling, 14-year-old Brandon Keith Warren, was shot one time with a self-inflicted gunshot wound to the roof of his mouth. Authorities found a .22 caliber semi-automatic rifle near the body of Brandon Warren. His body was also found in the middle bedroom.
Officials say they feel confident that the incident was a case of murder-suicide.
"The official crime scene was investigated by sheriff detectives and SBI agents, and all of the evidence and observations made and collected at the scene are consistent with this being a case of murder-suicide," says Capt. Dale Wheeler of the Johnston County Sheriff's Department.
Two of the bodies were naked, but Wheeler would not say which ones, and whether that was a factor in the case. Neighbors say social workers had come to the mobile home and the two brothers had been staying at a neighbor's home.
Detectives say the semi-automatic rifle used in the shooting belonged to the parents, Nissa and Kent Warren. Neither have been charged in the incident.
Police are still investigating what triggered the violence.
Services for the three teenagers will be held at 2 p.m. Saturday at McLaurin Funeral Home near Clayton. The memorial service is open to the public.
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