A 14-year-old Texas girl who shot a fellow student before turning her gun on herself last week planned to kill her stepbrother at their school, investigators now say.

The freshman girl, whose name has not been released, died of a self-inflicted gunshot wound in a restroom at Alpine High School. A 17-year-old junior was shot after she stumbled upon the armed, younger girl in the restroom.

ABC News reported that Alpine Police Chief Russell Scown gave a statement Tuesday offering additional details of the shooting. Scown said the girl sneaked a 9 mm semi-automatic handgun, 18 rounds of ammunition and a magazine into the school in her backpack.

"Investigators have determined that she intended to shoot her 14-year-old stepbrother and then shoot herself while inside the school. Her plan was interrupted when a 17-year-old female, a junior student at the school, entered the restroom," Scown said, according to ABC News.

The older girl ducked and ran after seeing the girl with the gun and was struck with a bullet as she fled. The shooter then shot and killed herself.

Scown did not say why the girl targeted her stepbrother or how investigators learned of her plans.

Alpine is the county seat of Brewster County, located in west Texas.