Colorado Springs police spokesman Larry Herbert said the girl's stepfather fired the shot, but said it wasn't known if he had mistaken the teen for a burglar.
Police say the girl was shot around 6 a.m. Monday and later died at the hospital.
The stepfather hasn't been arrested or identified and it will be up to prosecutors to decide whether charges should be filed.
Herbert said police are looking at the stepfather's intent and whether he acted recklessly as they investigate the shooting.
Colorado's "Make My Day" law allows residents to defend themselves against intruders.