At least 28 people - including 20 young children - have been killed in one of the worst school shootings in US history.
The victims at Sandy Hook Elementary School in Newtown, Connecticut, are believed to include the school principal, teachers and young pupils aged between five and 10.
The gunman - described as masked and heavily armed - reportedly used a .223-calibre rifle, as well as two handguns - a Glock and a Sig Sauer.
Other guns reportedly found at the school are being traced by state and federal authorities to confirm ownership, according to a law enforcement official.
The shooter has been named as 20-year-old Adam Lanza.
A body found at Lanza's family home in Newtown is believed to be his mother Nancy, a teacher at the school.
The killer is thought to have driven his mother's car to Sandy Hook before carrying out the attack in a bulletproof vest.
His older brother, 24-year-old Ryan Lanza is being questioned by police, but is not thought to have any involvement in the killings and was said to have been "extremely cooperative".
An official briefed on the investigation told the AP news agency that Mr Lanza has said his brother was believed to suffer from a personality disorder and be "somewhat autistic".
Hundreds of people have packed a church at Newtown's St Rose of Lima church for a memorial service, with many more spilling onto the street, where they held hands in circles and said prayers.
Governor Dannel Malloy said: "Evil visited this community today. It's too early to speak of recovery."
Lieutenant Paul Vance told a media conference earlier that 18 children had died at the school, two were taken to hospital for treatment where they later died. Six adults were found dead at the scene.
"The shooter is deceased inside the building," he added.
It has also been reported that the school headteacher has been killed.
One mother, Brenda Lebinski, told Fox News: "I talked to some parents that I knew and they said there were children being carried out all bloody, they thought they were dead.
"I was in total panic, numb, until I saw my daughter coming with her teacher from the school ... I grabbed her and it was the happiest moment of my life."
She added: "Another mum that I knew said she was actually in the school ... She said a masked gunman came in and was firing and that the principal is dead.
"This is a young vivacious woman who loved her job. She was the best principal."
Photos from the scene showed children, some of them crying, being escorted by adults through a car park in a line , hands on each other's shoulders.
Stephen Delgiadice said his eight-year-old daughter heard two big bangs before teachers told her to get in a corner. His daughter was unharmed.
"It's alarming, especially in Newtown, Connecticut, which we always thought was the safest place in America," he said.
US president Barack Obama, who ordered White House flags to be flown at half mast, wiped away tears as he paid tribute to the victims.
He said: "There is not a parent in America who does not feel the same overwhelming grief as I do."
PM David Cameron said he was "shocked and deeply saddened" by the tragedy and the Queen sent a message of condolence to President Obama.
Newtown is a small, tranquil community in one of the wealthiest counties in the US, about 60 miles (96km) northeast of New York City.
The incident began around half an hour after the school day had started.