At least 28 people – including 20 young children – have been killed in one of the worst school shootings in US history.
The death toll at Sandy Hook Elementary School, is believed to include the school principal, teachers, the gunman’s mother and young pupils aged between five and 10-years-old.
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.
He has been named by Associated Press at 20-year-old Adam Lanza.
His older brother, 24-year-old Ryan Lanza, is reportedly in police custody.
The school is in the south west of Connecticut
CBS news has said that a body has been found at an address linked to the gunman. It is understood the dead person may be his father, and that his mother Nancy Lanza, who was a teacher at the school, was also killed.
It is believed that most of the fatalities happened in her classroom.
Lanza’s girlfriend and another friend have also been reported missing in New Jersey.
US president Barack Obama 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.”
Mr Obama ordered flags at the White House to be flown at half mast.
Lieutenant Paul Vance told a media conference 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.
Meanwhile, local newspaper the Hartford Courant said an entire classroom was unaccounted for. It reported sources as saying many of the shootings took place in a kindergarten classroom.
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.”
Relatives wait for news
Staff at Danbury Hospital said they did not expect to receive any more patients than the three they were treating.
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.
White House spokesman Jay Carney said President Barack Obama was monitoring the situation.
“I think it’s important on a day like today to view this, as I know the president as a father does, and I as a father and others who are parents certainly do, which is to feel enormous sympathy for families that are affected,” he said.
The local mayor, Mark Boughton, posted on his Twitter page: “A horrible, horrible day for the Greater Danbury Community. Prayers for Newtown and the families of the children. #tragic.”
Local media initially reported there were two shooters, while police reportedly said a gunman was in the main office of the school and that a person in one room had “numerous gunshot wounds”.
The incident began around half an hour after the school day had started.
A dispatcher at the Newtown Volunteer Ambulance Corps said a teacher was shot in the foot and taken to Danbury Hospital.
But State Police are reportedly saying that the school is no longer an “active shooting scene”.
Crying pupils were seen being escorted from the school, which has children aged between five and 11 years old.