A gunman has killed 27 people, including 20 young children, and then himself 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 an emotional briefing from the White House.
He said the children killed in the attack at Sandy Hook Elementary School in Newtown, Connecticut, were aged between five and 10 years old.
He said the nation had been "through this too many times" and has to come together to take meaningful action, "regardless of the politics".
State police said another person had been found dead at a second scene.
A law enforcement official said the school shooting suspect, named as 24-year-old Ryan Lanza, died from a self-inflicted gunshot wound, and that his younger brother was being held for questioning as a possible second gunman.
He said the men's mother, Nancy Lanza, worked at the school as a teacher and was presumed dead.
Robert Licata said his six-year-old son was in class when the gunman burst in and shot the teacher.
"That's when my son grabbed a bunch of his friends and ran out the door," he said. "He was very brave. He waited for his friends."
Mr Licata said the gunman did not say a word.
The law enforcement official also said Ryan Lanza's girlfriend and another friend were missing in New Jersey.
According to the official, the suspect drove to the school in his mother's car.
Three guns were found - a Glock and a Sig Sauer, both pistols, and a .223-calibre rifle. The rifle was recovered from the back of a car at the school, and the two pistols were recovered from inside the school.
State police said only students and school staff were killed, but they refused to say how people were dead. Police said the scene was secure.
The attack, just two weeks before Christmas, was the latest of several mass shootings in the US this year, and approached the deadly scale of the Virginia Tech university massacre in 2007 in which 32 died.
Today, many of the 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.
The shooting shocked the small, tranquil community in one of the wealthiest counties in the US, about 60 miles (96km) north-east of New York City.
The last news items posted before the shooting on the website of the tiny newspaper, The Newtown Bee, lamented cracked headstones at a local cemetery and asked residents to "share 2012 memories".
Anguished parents came running when they heard news of the shooting.
Stephen Delgiadice said his eight-year-old daughter heard two big bangs, and teachers told her to get in a corner. His daughter was fine.
"It's alarming, especially in Newtown, Connecticut, which we always thought was the safest place in America," he said.
Mergim Bajraliu, 17, heard the gunshots echo from his home and raced to check on his nine-year-old sister at the school.
He said his sister, who was fine, heard a scream come over the intercom at one point. He said teachers were shaking and crying as they came out of the building.
"Everyone was just traumatised," he said.
Richard Wilford said his seven-year-old son, Richie, said he heard a noise that "sounded like what he described as cans falling".
The boy told him a teacher went out to check on the noise, came back in, locked the door and got the children to huddle in a corner until police arrived.
"There's no words," Mr Wilford said. "It's sheer terror, a sense of imminent danger, to get to your child and be there to protect him."
Melissa Makris said her 10-year-old son, Philip, saw what looked like a body under a blanket as he fled the school.
Earlier this year, a gunman killed 12 people at a Colorado cinema, and another gunman killed six people before killing himself at a Sikh temple in Wisconsin.
"We have endured too many of these tragedies," Mr Obama said. He was speaking in the James S Brady Press Briefing Room, named in honour of the former White House press secretary who was shot and disabled in the assassination attempt on Ronald Reagan in 1981.
Mr Brady and his wife, Sarah, have become activists for gun control measures.
"If now is not the time to have a serious discussion about gun control and the epidemic of gun violence plaguing our society, I don't know when is," one member of Congress, Jerrold Nadler, said in a statement. There was some confusion over the identity of the dead suspect at the school and the brother being questioned on suspicion of being a second gunman.
A law enforcement official said the dead suspect is 20-year-old Adam Lanza, the son of a teacher at the school where the shootings occurred.
A second law enforcement official said his mother, Nancy Lanza, is presumed dead.
The first official said Adam Lanza's older brother, 24-year-old Ryan, of Hoboken, New Jersey, is being questioned by police.
An earlier report from a law enforcement official mistakenly transposed the brothers' first names.
The first official said Adam Lanza died from a self-inflicted gunshot wound.