EVIDENCE has been found that could help explain the motives of the lone gunman behind one of the worst mass school shootings in American history, police said today.
Officers found evidence at Sandy Hook Elementary School in Newtown, Connecticut, where 20 children and six adults were shot and killed, and at a second crime scene where a woman was found dead, Lt Paul Vance told a press conference.
The gunman is believed to be 20-year-old Adam Lanza, who killed his mother Nancy at their home before going on the rampage at the school, although Lt Vance refused to confirm the gunman's identity.
"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 said.
He also revealed the gunman forced his way into the school.
Lt Vance added: "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."
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.
Lt Vance said that the identity of those killed had been confirmed and that they would be released as soon as possible.
Those believed dead include principal Dawn Hochsprung, who had been in charge since 2010.
A tearful President Barack Obama said: "Our hearts are broken today."
In an emotional televised address from the White House, he said: "We endured too many of these tragedies in the past few years. And each time I learn the news, I react not as a president, but as anybody else would - as a parent. And that was especially true today. I know there is not a parent in America who doesn't feel the same overwhelming grief that I do.
"The majority of those who died today were children. Beautiful little kids between the ages of five and 10 years old."
The president paused for some seconds and wiped away tears before continuing:
"Our hearts are broken for the parents of the survivors as well, for, as blessed as they are to have their children home tonight, they know that their children's innocence has been torn away from them too early, and there are no words that will ease their pain."
Lanza and his mother lived in a prosperous part of Newtown, a town with a population of 27,000 people about 60 miles north east of New York City.
Investigators are trying to learn as much as possible about him. Witnesses said Lanza did not speak during the rampage.
His older brother Ryan, 24, of Hoboken, New Jersey, who is being questioned by police but is not believed to have been involved, told investigators Adam, who lived with their mother in Connecticut, was believed to suffer from a personality disorder and be "somewhat autistic".
Adam's girlfriend and another friend are still missing in New Jersey, a police source told Associated Press.
Two pistols - a Glock and a Sig Sauer - were found inside the school, while a .223-calibre rifle was recovered from the back of a car at the site.
MaryAnne Jacobs, assistant librarian at the school, told today how she hid with 18 first grade children in a storage cupboard to avoid the gunman.
She said she followed the usual drill for emergency procedures, telling the children to be quiet and sit down before taking them initially to a place they thought safe and then to the room normally used to house servers for the library computers.
"We just told them it was a drill, they knew what to do," she said, fighting back tears.
"We knew it was gunshots first all because we made the phone call. I think they were like 'what is that? What is going on' but I don't think they really had any idea.
She added: "They weren't crying.
"We had to crawl out of the space we were in because we discovered one of our doors was unlocked and we locked ourselves in a back storage room where we have some (computer) servers inside the library.
"At that point they were a little freaked out I think, there were some crayons and paper so we tore up paper and gave them all something to do. It was hard to just keep them quiet and calm."
She said they barricaded the door with filing cabinets, only opening it when a police officer slid an identification badge underneath.