Twenty-seven people, including 20 children, were killed Friday when a gunman clad in black military gear opened fire inside Sandy Hook Elementary School in Newtown, Conn., the Associated Press reports. It is the nation's second-deadliest school shooting, exceeded only by the Virginia Tech massacre in 2007. Eighteen children -- all between the ages of 5 and 10 -- and six adults died at the scene. Two more children died at an area hospital, and another adult died at a location nearby. According to officials, the shooter, 20-year-old Adam Lanza, walked into the school shortly after 9 a.m.; the first 911 calls started ringing in the Newtown emergency service department around 9:30, and local officials quickly requested help from state police. Witnesses described hearing loud popping noises as he fired at students, teachers and administrators, and adults cowering in nearby classrooms counted as many as 100 shots. Connecticut State Police Lt. Paul Vance confirmed all of the shooting took place in two rooms, in one section of the school. "This is a tragedy of unspeakable terms," Gov. Dan Malloy said. "You can never be prepared for this kind of incident. What happened will leave a mark on this community and every family impacted." Sandy Hook Elementary School has around 600 students between kindergarten and fourth grade.