The gunman behind one of the worst mass school shootings in American history forced his way into the building, police have said, as they revealed they had found evidence that could explain his motives.
Adam Lanza, 20, is believed to be the man behind the massacre at Sandy Hook Elementary School in Newtown, Connecticut on Friday, which left 20 young children and six adults dead.
Lanza, described as suffering from a personality disorder and being "somewhat autistic" by his older brother Ryan, 24, had already killed their mother Nancy at their home in the town, and shot himself after the slaughter in the school, which he carried out using three guns.
The bloodbath brought despair and horror to a smalltown US community, 60 miles north-east of New York City, preparing for the Christmas holidays.
State Police Lieutenant Paul Vance told a press conference: "Our investigators at the crime scene - the school - and secondarily at the secondary crime scene we discussed, where the female was located deceased, did produce some very good evidence in this investigation that our investigators will be able to use in, hopefully, painting the complete picture as to how, and more importantly, why this occurred."
He also confirmed the gunman forced his way into the school, adding: "It is believed he was not voluntarily let into the school at all, that he forced his way into the school, but that is as far as we can go on that."
The mass killing has prompted fresh debate about the need for gun control in the US, as tales emerged of heroism by teachers and other staff to protect the children. Principal Dawn Hochsprung reportedly lunged at Lanza before being shot.
Maryann Jacob, who worked in the library, told how she led 18 children to safety by crawling with them to a storage room and waiting for the police to arrive. She said they barricaded the door with filing cabinets, only opening it when a police officer slid an identification badge underneath.
Lanza shot dead 18 children aged between five and 10 and six adults at the school where his mother Nancy was a teacher, before killing himself. Two other children shot at the scene died in hospital later. Friends and family members described the Newtown High School student variously as intelligent, nerdy, a goth and remote.
Lt Vance said that the identity of those killed had been confirmed and that they would be released as soon as possible. He said investigators were "peeling back the onion" of Lanza's life, including family and friends.