St. Louis boy, 11, left alone with sister shoots and kills persistent teen intruder (CNN)An 11-year-old left at home to defend himself and his 4-year-old sister staved off several home invasion attempts before finally shooting and killing a 16-year-old intruder, police say. Police officers arrived after 2 p.m. Thursday to the home on Hallwood Drive in north St. Louis County to discover the body of a 16-year-old lying in the front foyer. The boy, whose name has not been released, had been shot in the head while breaking into the home, according to police. He was shot by the 11-year-old boy who lived inside, and who -- along with his 4-year-old sister -- had been left there alone. Sgt. Brian Schellman said it was third time that the 16-year-old had attempted to break into that house that day. Neither sibling was hurt in the incident. It wasn't immediately known Thursday why the two were left alone in the first place or why an 11-year-old had access to a firearm, but police said the mother was cooperating with the investigation. CNN affiliate KTVI/KPLR reported the mother, whose name has not been released, purchased the handgun because of prior attempted break-ins. It is the third deadly incident involving children and guns in that section of St. Louis in recent weeks. On August 18, 9-year-old Jamyla Bolden was shot and killed while doing homework while lying in her mother's bed. A week later, 21-month-old Carter Epps died after being shot in the torso.