Police are examining the Bushmaster rifle Lanza used to shoot 20 children, four teachers, the school principal, a school psychologist at the Connecticut school, The Hartford (Conn.) Courant reported Saturday.
Before the shooting at Sandy Hook, Lanza shot and killed his mother, Nancy Lanza. He also fatally shot himself in the head as police arrived at the school.
Officials said based on reports from students who managed to escape from the first-grade classroom of teacher Victoria Soto, where Soto, special education teacher Anne Marie Murphy and six of Soto's 6- and 7-year-old students were fatally shot, investigators believe Lanza either mishandled or dropped a magazine and unfired bullets fell to the floor, or that the mechanism that fed bullets into the rifle jammed, causing Lanza to remove the magazine and clear the weapon. Either scenario would have given the students who fled the classroom enough time to do so.
Police are still trying to determine how many shots Lanza fired.
Authorities said they expect to wrap up the investigation at Sandy Hook within the next few days.