Twenty children and six adults have been killed in a shooting attack at a primary school in the US state of Connecticut, police say.
Police Lt Paul Vance said the gunman was also dead, but gave no cause.
US media identified the gunman, who died at Sandy Hook Elementary School, Newtown, as Ryan Lanza, 24, said to be from New Jersey.
The killings make Newtown the second-worst US shooting, after 32 died at Virginia Tech in 2007.
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.
According to US reports, the gunman's mother was a teacher found among the dead at the school. The Associated Press said his brother was being held by police.
The attacker was dressed in black and wearing a bullet-proof vest during the attack, and reportedly used a .223-calibre rifle. Other weapons are also said to have been recovered.
With the death toll rising, it emerged that one entire classroom of students may remain unaccounted for, local sources reported.
Three other people were taken to hospital and are reported to be in "very serious condition", Danbury Mayor Mark Boughton told CNN.
One witness speaking to CNN said that shots were heard coming from the hall. There "must have been 100 rounds" fired, she told the channel.
Local media have reported that firefighters instructed children to close their eyes and run past the school's office as they exited the building.
Other sources suggest that some of the shots were fired in a school classroom.
There were early unconfirmed reports of two shooters, but no further details of a second gunman mentioned by police.
Deadly US mass shootings
1984: James Oliver Huberty shoots dead 21 people at a McDonald's in California
1986: Postal worker Pat Sherrill kills 14 people at post office in Oklahoma
1991: George Hennard kills 23 people at a cafeteria in Texas
1999: Two students at Columbine high school kill 13 and injure 20, before killing themselves
2007: A student kills 32 and injures dozens more at Virginia Tech university
2009: 13 people are killed in a mass shooting at Ford Hood military base in Texas