The mother was presumed among the dead.
The killer, carrying two handguns, committed suicide at the school, and another person was found dead at a second scene, bringing the toll to 28, authorities said.
The rampage in the northeastern state of Connecticut was at least the fourth big shooting spree in five months in the United States. It was by far the deadliest of the year and most heart-wrenching.
The children were among the youngest victims of a mass shooting in recent history. Frightened students 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. The children killed were just 5 to 10 years old, he said. "They had their entire lives ahead of them -- birthdays, graduations, wedding, kids of their own."
The national debate over the issue of gun control in America exploded once again. Obama said the U.S. had been "through this too many times" with recent mass shootings and that the country has to come together to take meaningful action, "regardless of the politics." He did not give details.
Police shed no light on the motive for the attack. The gunman was believed to suffer from a personality disorder and lived with his mother in Connecticut, said a law enforcement official who was briefed on the investigation but was not authorized to publicly discuss it.
The attack at Sandy Hook Elementary School, just two weeks before Christmas, was the nation's second-deadliest school shooting, exceeded only by the Virginia Tech university massacre in 2007 that left 32 dead.
This time, the victims were far smaller. Photos from the scene showed students, some of them crying, being escorted by adults through a parking lot 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. Teachers ordered them to hide in closets or corners.
State police said 18 children were found dead at the school and two later were declared dead, and six adults were found dead at the scene. Police said another person was found dead at a second scene, leading to a total death toll of 28.
Gov. Dannel P. Malloy said someone who lived with the gunman died.
A law enforcement official said the suspect, 20-year-old Adam Lanza, was dead from a self-inflicted gunshot wound, and he was the son of a teacher at the school. A second law enforcement official said the mother, Nancy Lanza, was presumed dead.
Adam Lanza's older brother, 24-year-old Ryan, of Hoboken, New Jersey, was being questioned.
The law enforcement official who said Adam Lanza had a possible personality disorder said Ryan Lanza had been extremely cooperative, was not believed to have any involvement in the rampage and was not under arrest or in custody, but investigators were still searching his computers and phone records.
Ryan Lanza told law enforcement he had not been in touch with his brother since about 2010.
All three law enforcement officials spoke on the condition of anonymity because they were not authorized to speak on the record about the unfolding investigation.
The gunman drove to the school in his mother's car, the second official said. Three guns were found. Three guns were found -- a Glock and a Sig Sauer, both pistols, inside the school, and a .223-caliber rifle in the back of the car.
The official also said Lanza's girlfriend and another friend were missing in New Jersey.
The shooting shocked a small, tranquil community in one of the wealthiest counties in the U.S., about 60 miles (96 kilometers) northeast of New York City. 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 Friday morning.
Robert Licata said his 6-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."Licata said the shooter didn't say a word.
Stephen Delgiadice said his 8-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 ran to check on his 9-year-old sister, who was fine. He said she 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 traumatized," Bajraliu said.
Richard Wilford said his 7-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 had the kids huddle in the corner until police arrived."There's no words," Wilford said.
Melissa Makris said her 10-year-old son, Philip, saw what looked like a body under a blanket as he fled the school.
The shootings instantly brought to mind episodes such as the Columbine school massacre in Colorado that killed 15 in 1999.
"I think as a society, we need to come together. It has to stop these senseless deaths," Columbine principal Frank DeAngelis said Friday.
Already this year, a gunman killed 12 people at a Colorado theater, and another gunman killed six people before killing himself at a Sikh temple in Wisconsin.
"We have endured too many of these tragedies," Obama said.
He addressed reporters in the James S. Brady Press Briefing Room, named in honor of the former White House press secretary who was shot in the assassination attempt on Ronald Reagan in 1981. 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, Rep. Jerrold Nadler, said in a statement.
Overseas, there was both shock and sympathy.
In a public statement addressed to Obama, French President Francois Hollande said he was "horrified."
British Prime Minister David Cameron said "it is heartbreaking to think of those who have had their children robbed from them." Queen Elizabeth II sent a message Obama saying she was saddened by the "dreadful loss of life."
A law enforcement official said, the brother of the gunman has been "extremely cooperative" and is not under arrest.
The official told AP that Ryan Lanza, of Hoboken, New Jersey, is still be being questioned but is not in custody and is not believed to have any connection to the school killings.
The official said, Ryan Lanza's computers and phone records were being searched but only "in an abundance of caution." He says Ryan told authorities he had not been in touch with his brother in recent years.
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