Investigators were trying to learn more about the gunman, Adam Lanza, and questioned his older brother, who is not believed to have been involved in the rampage at Sandy Hook Elementary. Police shed no light on the motive for the nation's second-deadliest school shooting.
In tight-knit Newtown on Friday night, hundreds of people packed St. Rose of Lima church and stood outside in a vigil for the 28 dead - 20 children and six adults at the school, the gunman's mother at home, and the gunman himself, who committed suicide. People held hands, lit candles and sang "Silent Night."
"These 20 children were just beautiful, beautiful children," Monsignor Robert Weiss said. "These 20 children lit up this community better than all these Christmas lights we have. ... There are a lot brighter stars up there tonight because of these kids."
Lanza is believed to have suffered from a personality disorder and lived with his mother, said a law enforcement official who was briefed on the investigation.
Lanza shot his mother, Nancy Lanza, drove to the school in her car with at least three guns, including a high-powered rifle that he apparently left in the back of the vehicle, and shot up two classrooms around 9:30 a.m. Friday, law enforcement officials said, speaking on condition of anonymity.
A custodian ran through the halls, warning of a gunman, and someone switched on the intercom, perhaps saving many lives by letting them hear the chaos in the school office, a teacher said. Teachers locked their doors and ordered children to huddle in a corner or hide in closets as shots echoed through the building.
The well-liked principal, Dawn Hochsprung, was believed to be among the dead. A woman who worked at the school was wounded.
A law enforcement official speaking on condition of anonymity said investigators believe Lanza attended the school several years ago but appeared to have no recent connection to it.
At least one parent said Lanza's mother was a substitute teacher there. But her name did not appear on a staff list. And the official said investigators were unable to establish any connection so far between her and the school.
Lanza's older brother, 24-year-old Ryan Lanza, of Hoboken, N.J., was questioned, but a law enforcement official said he was not believed to have had a role in the rampage. Investigators were searching his computers and phone records, but he told law enforcement he had not been in touch with his brother since about 2010.
The officials spoke on the condition of anonymity because they were not authorized to discuss the unfolding investigation.
At one point, a law enforcement official mistakenly identified the gunman as Ryan Lanza. Brett Wilshe, a friend of Ryan Lanza's, said Lanza told him the gunman may have had his identification. Ryan Lanza apparently posted Facebook page updates Friday afternoon that read, "It wasn't me" and "I was at work."
For about two hours late Friday and early Saturday, clergy members and emergency vehicles moved steadily to and from the school. The state medical examiner's office said bodies of the victims would be taken there eventually for autopsies.
At least 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, authorities said. A law enforcement official speaking on condition of anonymity said some of the guns used in the attack may have belonged to Lanza's family. His mother had legally registered four weapons, his father two.
Authorities also recovered three other guns - a Henry repeating rifle, an Enfield rifle and a shotgun. It was not clear exactly where those weapons were found.
Adam Lanza and his mother lived in a well-to-do part of prosperous Newtown, about 60 miles northeast of New York City, where neighbors are doctors or hold white-collar positions at companies such as General Electric, Pepsi and IBM.
Lanza's parents filed for divorce in 2008, according to court records. His father, Peter Lanza, lives in Stamford, Conn., and works as a tax director for General Electric.
The gunman's aunt Marsha Lanza, of Crystal Lake, Ill., said her nephew was raised by kind, nurturing parents who would not have hesitated to seek mental help for him if he needed it.
"Nancy wasn't one to deny reality," Marsha Lanza said, adding her husband had seen Adam as recently as June and recalled nothing out of the ordinary.
Catherine Urso, of Newtown, said her college-age son knew the killer and remembered him for his alternative style. "He just said he was very thin, very remote and was one of the goths," she said.
Adam Lanza attended Newtown High School, and several news clippings from recent years mention his name among the honor roll students.
Joshua Milas, who graduated from Newtown High in 2009 and belonged to the school technology club with him, said Adam Lanza was generally a happy person but that he hadn't seen him in a few years.
"We would hang out, and he was a good kid. He was smart," Joshua Milas said. "He was probably one of the smartest kids I know. He was probably a genius."
An official who spoke on condition of anonymity said it was not clear that Adam Lanza had a job, and there was no indication of law enforcement interviews or search warrants at a place of business.
The mass shooting is one of the deadliest in U.S. history, and among school attacks is second in victims only to the 2007 Virginia Tech massacre, which left 33 people dead, including the gunman. Reaction was swift and emotional in Newtown, a picturesque New England community of 27,000 people, as well as across the country and around the world.
"It has to stop, these senseless deaths," said Frank DeAngelis, principal of Colorado's Columbine High School, where a massacre in 1999 killed 15 people.
In Washington, the Coalition to Stop Gun Violence organized a vigil at the White House, with some protesters chanting, "Today IS the day" to take steps to curb gun violence. In New York's Times Square, a few dozen people held tea lights in plastic cups, with one woman holding a sign that read: "Take a moment and candle to remember the victims of the Newtown shooting."
President Barack Obama's comments on the tragedy amounted to one of the most outwardly emotional moments of his presidency.
"The majority of those who died were children - beautiful little kids between the ages of 5 and 10 years old," Obama said at a White House news briefing. He paused for several seconds to keep his composure as he teared up and wiped an eye. Nearby, two aides cried and held hands.
Australian Prime Minister Julia Gillard described the attack as a "senseless and incomprehensible act of evil."
"Like President Obama and his fellow Americans, our hearts too are broken," Gillard said in a statement.
In Japan, where guns are severely restricted and there are extremely few gun-related crimes, the attack led the news two days before nationwide parliamentary elections. In China, which has seen several knife rampages at schools in recent years, the attack quickly consumed public discussion.
At the Newtown vigil, Anthony Bloss, whose three daughters survived, said they are doing better than he. "I'm numb. I'm completely numb," he said.
Panicked parents looking for their children had raced earlier in the day to Sandy Hook, a kindergarten-through-fourth-grade school where police told youngsters to close their eyes as they were led from the building so that they wouldn't see the blood and broken glass.
Schoolchildren - some crying, many looking frightened - were escorted through a parking lot in a line, hands on one another's shoulders.
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.
Kaitlin Roig, a teacher at the school, said she implored her students to be quiet.
"I told them we had to be absolutely quiet. Because I was just so afraid if he did come in, then he would hear us and just start shooting the door. I said we have to be absolutely quiet. And I said there are bad guys out there now and we need to wait for the good guys to come get us out," Roig told ABC News.
"If they started crying, I would take their face and say it's going to be OK. Show me your smile," she said. "They said, we want to go home for Christmas. Yes, yeah. I just want to hug my mom, things like that, that were just heartbreaking."
Contributing to this report were Associated Press writers Pat Eaton-Robb and Matt Apuzzo and videographer Robert Ray in Newtown; Bridget Murphy in Boston; Samantha Henry in Newark, N.J.; Pete Yost in Washington; Michael Melia in Hartford; and the AP News Research Center in New York.
RELATED: More coverage on ABCNews.com