Evidence of gunman's motives found in Connecticut mass shooting investigation as police say he 'forced his way into school'

Principal Dawn Hochsprung reportedly lunged at Lanza before being shot

Clockwise from top right: White balloons decorate the sign for the Sandy Hook Elementary School, alleged gunman Adam Lanza, a woman comforts a young girl during a vigil service and Sandy Hook principal Dawn Lafferty Hochsprung, who was killed in the shooting (AP; Rex Features)

The gunman behind one of the worst mass school shootings in American history forced his way into the building, police said today, 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 yesterday, 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 USA community, 60 miles north-east of New York City, preparing for the Christmas holidays.

State Police Lieutenant Paul Vance told a press conference this afternoon: "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 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.

Board of Education chairwoman Debbie Liedlien told Associated Press that administrators were coming out of a meeting when the gunman forced his way into the school, and they ran toward him.

Jeff Capeci, chairman of the town's legislative council, was asked whether Ms Hochsprung is a hero. He replied: "From what we know, it's hard to classify her as anything else."

Maryann Jacob, who worked in the library, told today 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 today described the Newtown High School student variously as intelligent, nerdy, a goth and remote.

His girlfriend and another friend are still missing in New Jersey, a police source told AP.

The attack was the latest of several mass shootings in the US this year and carries echoes of the Dunblane massacre, where 16 schoolchildren and one teacher were killed by gunman Thomas Hamilton in the Scottish town in 1996.

It approached the deadly scale of the Virginia Tech university massacre in 2007 that left 32 dead.

The United States also is still reeling from the "Batman" shootings in Aurora, Colorado, in June.

Alleged killer James Holmes, 24, is awaiting trial charged with 24 counts of murder and 116 counts of attempted murder after the cinema shootings which left 12 people dead and 58 wounded.

In another incident today a gunman was shot dead by police after opening fire in St Vincent's Hospital in Birmingham, Alabama, injuring an officer and two employees.

Two pistols - a Glock and a Sig Sauer - were found inside the Newtown school, while a .223-calibre rifle was recovered from the back of a car at the site and officers are currently determining whose they are.

Lt Vance said that the identity of those killed had been confirmed and that they would be released as soon as possible.

Condolences flooded in for those killed, led by the Queen and Prime Minister David Cameron in the UK, with others from figures including Pope Benedict XVI.

President Barack Obama had made a televised address to the nation on Friday night, saying "our hearts are broken today".

The same night hundreds of people packed the town's St Rose of Lima church and stood outside in a vigil for the 28 dead in total, including the gunman.

Lt Vance said investigators were "peeling back the onion" of Lanza's life, including family and friends.

"We still have major crime detectives and Newtown detectives working at the scene in the school," he continued.

"That is not completed, that probably will not be completed for at least another day-and-a-half to two days. I'm not putting a time limit on it, it could take longer.

"It's going to be a long, painstaking process."

PA