The investigation included a secondary crime scene in Connecticut and searches in New Jersey where Lanza and his relatives had lived. Lanza's brother, Ryan, who had originally been identified as the gunman, was being questioned by police. Officials later said Ryan Lanza was not believed to be involved in the shootings.
FULL COVERAGE: Connecticut school shooting
By late in the day, Connecticut State Police spokesman Paul Vance said at a televised news conference that the number of deaths overall stood at 28. One person was injured.
The victims included 18 children and six adults pronounced dead at the school and two pupils pronounced dead at hospitals. The gunman, who was found dead at the school, and another person dead at the secondary crime scene brought the number of bodies to 28, the deadliest school attack since the 2007 rampage at Virginia Tech.
But unlike the Virginia Tech rampage, during which 33 people including the gunman died, the Connecticut shootings were at an elementary school where pupils ranged from kindergarten to fourth grade.
The youth of the victims tugged at the nation, which was led in mourning by President Obama.
“We've endured too many of these tragedies in the past few years, and each time I hear the news, I react not as a president but as everybody else would, as a parent,” Obama said, wiping tears from his eyes during a televised appearance at the White House. “I know there's not a parent in America who doesn't feel the same overwhelming grief that I do.
“Our hearts are broken today for the parents and grandparents, sisters and brothers of these little children and for the families of the adults who were lost,” he told the nation. “Our hearts are broken for the parents of the survivors as well, for as blessed as they are to have their children home tonight, they know that their children's innocence has been torn away from them too early and there is no way to ease their pain.”
Until the terror began unfolding, it had been a typical Friday in this picturesque, bucolic town of rambling Victorian-style homes. Newtown, about 60 miles from New York City, is part of Fairfield County in western Connecticut. There's a giant tree decked out in holiday blue lights at the entrance to the town and many homes are displaying wreaths and festive lights.
"Evil visited this community today," Gov. Dannel P. Malloy said at an evening news conference. "It's too early to speak of recovery."
A fifth-grade teacher who gave her name only as Dina, said earlier, Newtown is "such a tight-knit community. Everybody knows everybody and it's such a shock. This is the hardest day of my career."
About 9:30 a.m, the gunman entered Sandy Hook Elementary School, where sources said he had a confrontation with a school official. Then the gunman opened fire in just one section of the school, two rooms, police spokesman Vance said at one of his news conferences later in the day.
A third-grader named Alexis told CNN she looked out the window, spotted police officers and heard footsteps on the school roof. Some of the students were so upset, she said, “they got a stomachache.”
A fourth-grade boy was in the gym when he heard pops and bangs. “We thought it was the custodian knocking stuff down,” he told CNN. “We heard screaming … and then the police came in and said, ‘Is he in here?’”
The children were herded into a gym closet, the fourth-grader said, and huddled there until police told them it was safe to leave.
The first calls went out to police by 9:40 a.m. and hundreds of officers from the town police, state police and others responded, authorities said. They began a painstaking search through the building, room by room.
Panicked parents were caught on camera racing to the school and filling the parking lot as some children were escorted through the line. Children placed their hands on each other's shoulders, keeping in a tight formation.
The earliest reports indicated just a few fatalities, but Danbury Mayor Mark D. Boughton signaled to reporters that the disaster was much worse than that. “There had been multiple fatalities,” he told television viewers, then declined to give details.
Law enforcement sources in Washington and elsewhere then told reporters that, indeed, the toll had risen. For the next several hours, the numbers climbed until police confirmed later in the day that 28 were dead in all.
Confusing things was a mistaken identity of the shooter. Police sources finally said it was Adam Lanza who was the gunman while his brother, Ryan, was being questioned. Their mother, Nancy, worked at the school with kindergarten children.
Late Friday, authorities confirmed that the body of Lanza's mother, Nancy, was found at the secondary crime scene.
Georgeann Manfredonia, a volunteer at the school, said she spent part of Thursday wrapping presents for the teachers and the school's principal. Her 6-year-old daughter attends the Sandy Hook.
“When you walk in there, it’s just a happy, happy place,” she said, in a telephone interview Friday with The Los Angeles Times. “It’s a wonderful school run by an amazing principal. She’s just the most positive, energetic person you’ve ever met.
“You always felt safe here. The last crime that took place was years ago,” Manfredonia said.
On Friday morning, her husband, Chris, went to help make gingerbread houses in the first-grade class of the couple’s daughter. As he approached the school, he heard popping sounds, then smelled sulfur.
He attempted to reach his daughter by going around the side of the school. Police had arrived by then, stopped him and briefly handcuffed him.
Georgeann found a neighbor’s child and held that child and her own son. A parent came running out of the school cradling a little girl.
“I covered them so they couldn’t see anything," she said. "I couldn’t see if the little girl was hurt.”
News still had not filtered out about the shooting and few parents were around. Police and firefighters began leading the children from the school to a nearby firehouse.
Susan Birge, trained as a mental health disaster responder for the Red Cross, spent the afternoon counseling the 15 to 20 family members huddled in the firehouse. "They are just devastated," she said.
Because the crime scene is so extensive, victims' families will not be able to see their loved ones until tomorrow at the earliest, she said. Instead, they're in the firehouse with food, clergy and each other.
Standing outside the firehouse, where officials were delivering terrible news to parents, Msgr. Robert Weiss of St. Rose Church said he had been comforting grieving families.
“I just had a situation where a mother’s alarm went off and it was a notice to take her son to Cub Scouts,” Weiss said. “And then she realized she won’t be taking him to Cub Scouts anymore.”
Weiss said he had to tell the boy’s sister that she would not be able to play with her brother again.
A memorial for victims was scheduled Friday evening at St. Rose. Another memorial was planned for Trinity Church.
The shooting is just the latest to stun a nation still reeling from such tragedies as the 1999 Columbine High School massacre that left 15 dead. Last summer, more than 12 were killed when a gunman opened fire in a movie theater in Aurora, Colo. And earlier this week, three, including the gunman, died in a shooting at a shopping mall in Portland, Ore.
The only mass shooting in the United States with more than 27 killed since the 1950s took place on April 16, 2007, when student Seung-Hui Cho shot and killed 32 people at Virginia Tech in Blackburg, Va., before shooting himself.
Susman reported from Connecticut, Serrano from Washington and Muskal from Los Angeles.
Times staff writers Sam Quinones, Robin Abcarian, Alana Semuels, Ashley Powers, Andrew Khouri, Marisa Gerber and Matt Pearce contributed to this report.