The motive behind Sandy Hook Elementary School Massacre

Police in U.S. state of Connecticut on Saturday said the shooter of Sandy Hook Elementary School massacre forced his way into the school before Friday's rampage, as authorities and the world wonder about the motives behind such senseless killings.

Lieutenant Paul Vance, spokesman for the state police, told a press conference here that evidence collected from the ongoing investigation indicated the shooter forced his way into the school, not voluntarily let in.

Press reports said investigators found shattered glass on the scene, indicating the shooter shot his way into the school, not buzzed in as previous reports have suggested. The school had installed new security system not long before the shooting. Under the system, every visitor was required to ring a doorbell at the front entrance after the doors locked at 9:30 a.m. and report to the main office to sign in.

Vance didn't release the names of victims or the shooter during his short press conference, saying he was waiting for the medical examiner to finish determining their identities.

A heavily armed gunman, identified by press reports as 20-year- old Adam Lanza, on Friday morning forced his way into the school, and started a shooting rampage, killing 20 children and six adults. He later killed himself. His mother, identified as Nancy, was found dead at her home not far from the school. Nancy Lanza was said to be a teacher or worker at the school, but the information was not confirmed. The motives behind the killings remain unclear at the moment.