Newtown school gunman forced his way in, police say
The gunman who killed 20 children and six adults at a school in Newtown, Connecticut, forced his way into the building, police say.
The suspect, named in media reports as Adam Lanza, was not "voluntarily let in", said Lt Paul Vance.
He said police had found "very good evidence" which would help them establish a motive.
The only person hurt in the attack at Sandy Hook Elementary School would be "instrumental" in the inquiry, he said.
The gunman killed his mother at the home they shared before driving to the school in her car and opening fire. Reports say the guns used in the attacks were registered to her.
President Obama has urged "meaningful action" against gun crime in the US in the wake of the attack.
"As a country we have been through this too many times," he said in an emotional White House address.
"We're going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics."
The children killed at Sandy Hook Elementary were aged between five and 10, although the exact ages of the victims have not been revealed.
All the victims have now been identified, Lt Vance said, and their names would be released later. Principal Dawn Hochsprung is believed to be among them.
Initial reports suggested the gunman's mother, believed to be Nancy Lanza, may have worked at the school.
However, school superintendent Janet Robinson told NBC News on Saturday: "We have no Nancy Lanza in our database. She may have been a substitute teacher, but in terms of being a full-time employee, she's not."
Friday's killings took place in two rooms within a single section of the school, police have said. The shooting lasted just a few minutes.
As they heard the shots, teachers in other parts of the building tried to protect children by locking doors and ushering them into closets.
"I told them we had to be absolutely quiet, because I was just so afraid if he did come in, then he would hear us and just start shooting the door," said teacher Kaitlin Roig.
"I said to them, 'I need you to know that I love you all very much and that it's going to be okay', because I thought that was the last thing they were ever going to hear."
Library clerk Maryann Jacob described telling 18 children to crawl into a storage room, before barricading the door with a filing cabinet. "We set them up with paper and crayons," she said.
Dr Jeannie Pasacreta, a nurse practitioner and psychologist who has been advising parents on how to talk to their children, says neighbours have been cancelling Christmas parties and taking down decorations.
Officials say Ms Hochsprung was killed while lunging at the gunman, AP news agency reported. Board of Education chairwoman Debbie Liedlien said administrators were emerging from a meeting when the gunman forced his way into the school, and they ran toward them.
Early reports named 24-year-old Ryan Lanza, of Hoboken, New Jersey, as the gunman, but unnamed officials later said his brother Adam was the suspect.
Ryan Lanza was questioned by police, US media reported, but has not been named as a suspect.
In a separate development, police in Oklahoma have arrested a teenager for allegedly plotting to shoot and bomb students at his school in Bartlesville. Sammie Eaglebear Chavez, 18, is said to have tried to convince other students to help him carry out an attack.
The attack at Newtown is the second deadliest shooting attack at a US school or university.
In 2007, a student at Virginia Tech university killed 32 people and injured many more.