Police say the 19-year-old gunman who shot and killed two people at a St. Louis high school purchased the AR-15-style rifle from a private seller after an FBI background check stopped him from buying a weapon from a licensed dealer.

Relatives of the gunman who killed a student and a teacher during a St. Louis school shooting had long been concerned about his mental health and worked with police to take a gun away from him — possibly the same gun used in the attack.

Police say the 19-year-old gunman who killed a teacher and a 15-year-old girl at a St. Louis high school was armed with an AR-15-style rifle and what appeared to be more than 600 rounds of ammunition.

An armed former student broke into a St. Louis high school Monday morning, fatally shooting a teacher and a teenage girl and wounding seven others before police killed him.