The 20-year-old 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.
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 rampage, coming less than two weeks before Christmas, was the nation's second-deadliest school shooting, exceeded only by the Virginia Tech massacre that left 33 people dead in 2007.
Panicked parents looking for their children raced to Sandy Hook Elementary School in Newtown, a prosperous community of about 27,000 people 60 miles northeast of New York City. Youngsters at the kindergarten-through-fourth-grade school were told to close their eyes by police as they were led from the building.
Schoolchildren — some crying, others looking frightened — were escorted through a parking lot in a line, hands on each other's shoulders.
“Our hearts are broken today,” a tearful President Barack Obama, struggling to maintain composure, said at the White House. He called for “meaningful action” to prevent such shootings.
Youngsters and their parents described teachers locking doors and ordering the children to huddle in the corner or hide in closets when shots echoed through the building. Authorities didn't say exactly how the shootings unfolded.
They also gave no details on the victim discovered at another scene, except to say that the person was an adult found dead by police while they were investigating the gunman.
A law enforcement official identified the gunman as 20-year-old Adam Lanza, the son of a teacher. A second law enforcement official said his mother, Nancy Lanza, was presumed dead.
Adam Lanza's older brother, 24-year-old Ryan, of Hoboken, N.J., 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 — a Glock and a Sig Sauer, both pistols, inside the school, and a .223-caliber rifle in the back of a car.
State police Lt. Paul Vance said 28 people in all were killed, including the gunman, and one person was injured.
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.”
He said the shooter didn't utter a word.
Stephen Delgiadice said his 8-year-old daughter was in the school and heard two big bangs. Teachers told her to get in a corner, he said.