A gunman killed 26 people, including 20 young children, at a US school where his mother worked in one of the worst school shootings in the country's history.
Frightened pupils who were rushed from the building by police were told to close their eyes.
"Our hearts are broken today," President Barack Obama said, wiping his eyes during brief comments to reporters in one of the most emotional public moments of his presidency. He said the children killed were aged between five and 10.
He said the nation had been "through this too many times" with recent mass shootings and has to come together to take meaningful action, "regardless of the politics". He did not give details, even as the debate over the issue of gun control in America exploded once again.
A law enforcement official said the suspect, 20-year-old Adam Lanza, was dead from a self-inflicted gunshot wound, and was the son of a teacher at the school. A second law enforcement official said the mother, Nancy Lanza, was presumed dead. The first official said Adam Lanza's older brother, 24-year-old Ryan, was being questioned by police. An earlier report from a law enforcement official mistakenly transposed the brothers' first names.
The attack at Sandy Hook Elementary School, just two weeks before Christmas, was the latest of several mass shootings in the US this year, and it approached the deadly scale of the Virginia Tech university massacre in 2007 that left 32 dead.
This time, many victims were young children. Photos from the scene showed students, some of them crying, being escorted by adults through a car park in a line, hands on each other's shoulders. Children told their parents they had heard bangs and, at one point, a scream over the intercom.
State police said 18 children were found dead at the school and two died later in hospital. Six adults were found dead at the scene. They said the shootings occurred in one section of the school but did not give details. Police said another person was found dead at a second scene, leading to a total death toll of 28. Connecticut Governor Dannel Malloy said someone who lived with the gunman died.
According to the second law enforcement official, the suspect drove to the scene in his mother's car. Three guns were found - a Glock and a Sig Sauer, both pistols - and a .223-caliber rifle. The official also said Lanza's girlfriend and another friend are missing in New Jersey.
Police officials said later that Lanza's brother had been "extremely co-operative" and was not under arrest. Ryan Lanza, of Hoboken, New Jersey, was still being questioned but was not in custody and was not believed to have any connection to the killings. He reportedly told authorities he had not been in touch with his brother in recent years. His computers and phone records were being searched but only "in an abundance of caution", the officials said.