ST. LOUIS (AP) — 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, Police Commissioner Michael Sack said Tuesday.

Orlando Harris also left behind a hand-written note offering his explanation for the shooting Monday at Central Visual and Performing Arts High School. Tenth-grader Alexzandria Bell and 61-year-old physical education teacher Jean Kuczka died and seven students were wounded before police killed Harris in an exchange of gunfire.

Sack read Harris'...