A gunman has killed 20 children and six adults at a primary school in the US state of Connecticut, police say.
The gunman, who also died, has not been formally identified by police.
But officials told US media that the killer at Sandy Hook Elementary School, in Newtown, was a 20-year-old son of a teacher. He is thought to have killed her before the attack.
It is one of the worst-ever US school shootings, with a toll close to the 32 who died at Virginia Tech in 2007.
Early reports named 24-year-old Ryan Lanza as the gunman, but anonymous officials later said his brother Adam, 20, was the suspect.
He is believed to have killed their mother, Nancy Lanza, at her home before heading to Sandy Hook school. Investigators say it is unclear whether she worked there.
Ryan Lanza of Hoboken, New Jersey, was being questioned by police, US media reported, but has not been named as a suspect.
'Safest place in America'
Police Lt Paul Vance said 18 children were pronounced dead at the school, and two died after being taken to hospital. Six adults were also killed, and the gunman died at the scene, apparently after shooting himself.
One person was also injured, and police were investigating a second crime scene in Newtown, where another victim was found dead - understood to be the gunman's mother.
Dressed in black and wearing a bullet-proof vest, the gunman is thought to have had several weapons.
These included two handguns - a Glock and a Sig Sauer - and a .223-calibre rifle, reports said.
The killings took place in two rooms within a single section of the school, police have said.
One parent, Stephen Delgiadice, whose eight-year-old daughter was at Sandy Hook School on Friday but was not harmed, said the shooting was traumatic for the small town.
"It's alarming, especially in Newtown, Connecticut, which we always thought was the safest place in America," Mr Delgiadice told AP.
At a memorial service in Newtown people crowded outside the doors of the church as Connecticut Governor Dannel Malloy addressed those gathered.
He called the attack a "tragedy of unspeakable terms", saying "you can never be prepared" for an event like this.
In Washington, about 200 people held a candlelight vigil for the victims outside the White House, whilst others protested there to call for gun controls.
'Innocence torn away'
Friday's shooting is the third major gun attack in the US in 2012.
In July an attacker killed 12 people at a premiere of a Batman film in Aurora, Colorado. In August six people died at a Sikh temple in Wisconsin.
Just this week two people died in a shooting at a shopping mall in the state of Oregon.
At the White House, an emotional President Barack Obama cited those incidents as he called for "meaningful action... regardless of politics".
"Our hearts are broken today, for the parents, grandparents, sisters and brothers of these children, and for the families of the adults who were lost."
Mr Obama offered condolences to the families of survivors too, saying "their children's innocence has been torn away from them too early, and there are no words that will ease their pain".
He wiped tears from his eyes as he spoke of the "overwhelming grief" at the loss of life.
The American flags on Capitol Hill were lowered to half-mast in the wake of the attack.
Schools locked down
Police arrived at the school soon after 09:40 local time (14:40 GMT) on Friday, answering reports that a gunman was in the school's main office and one person had "numerous gunshot wounds".
Witnesses reported hearing scores of shots fired, with one suggesting there "must have been 100 rounds" in an interview with CNN.
Parent Richard Wilford said his seven-year-old son Richie described a noise that "sounded like what he described as cans falling".
Mr Wilford said his son's teacher locked the classroom door and told the children to huddle in a corner until the police arrived.
The authorities said they mobilised "every possible asset" in their response to the shooting. Teams of officers, some with dogs, combed the school and evacuated the building.
Firefighters reportedly told children to close their eyes and run past the school's office as they left the building.
Sandy Hook School - described by correspondents as a highly rated school has more than 600 students - spanning the ages five to 10.
Witness Mergim Bajraliu, 17, said he heard the gunshots ring out from his home and ran to the school looking for his nine-year-old sister, who was not hurt.
He spoke of an emotional scene at the school as anguished parents rushed to find their children.
"Everyone was just traumatised," he said.