The gunman who massacred 26 children and adults at a U.S. school forced his way in, police said Saturday, as a stunned world asked how a 20-year-old described as brilliant but remote would have been driven to such a crime and how he chose his victims, some as young as 5.

Witnesses said the gunman, Adam Lanza, didn't say a word as he burst into a classroom, shooting, and later killed himself. Connecticut state police Lt. Paul Vance said investigators had found "very good evidence" and hoped it would answer questions about the gunman's motives. Vance would not elaborate.

The bodies of victims were still inside the school for some time Saturday morning, and authorities appeared poised to start releasing their names.

The mass shooting was one of the deadliest in U.S. history, and among school attacks it was second only to the 2007 Virginia Tech massacre, which left 33 people dead. Reaction was swift and emotional around the world, any many immediately thought of Dunblane _ a 1996 shooting in that small Scottish town which killed 16 children and prompted a campaign that ultimately led to tighter gun controls.

President Barack Obama's comments on the tragedy were 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 told a White House news briefing. He paused for several seconds to keep his composure and wiped an eye. Nearby, two aides cried and held hands.

In tight-knit Newtown, a picturesque New England community of 27,000 people, hundreds of people packed St. Rose of Lima church Friday night 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.

Just 10 days before Christmas Eve, 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."

"People in my neighborhood are feeling guilty about it being Christmas. They are taking down decorations," said Jeannie Pasacreta, a psychologist who was advising parents on how to talk to their children.

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 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.

Maryann Jacob, a clerk in the school library, was with 18 students when they heard gunfire outside the room. She had the children crawl into a storage room, and they locked the door and barricaded it with a file cabinet. There happened to be materials for coloring, "so we set them up with paper and crayons."

After what she guessed was about an hour, officers came to the door and knocked, but those inside couldn't be sure.

"One of them slid his badge under the door, and they called and said, `It's OK, it's the police,'" Jacob said.

The well-liked principal, Dawn Hochsprung, was believed to be among the dead. A woman who worked at the school was wounded.

Adam Lanza attended Newtown High School, and several news clippings from recent years mention his name among the honor roll students.

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, New Jersey, 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.

A law enforcement official said a Glock and a Sig Sauer, both pistols, and a .223-caliber Bushmaster rifle were found in the school and a fourth weapon was found outside the school, and that investigators were going to shooting ranges and gun stores to see if Lanza had frequented them

The official was not authorized to discuss information with reporters and spoke on condition of anonymity.

Adam Lanza and his mother lived in prosperous Newtown, about 60 miles (95 kilometers) 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 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," she said.

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.

Meanwhile, a long-simmering national debate on gun control exploded again.

In Washington, the Coalition to Stop Gun Violence organized a vigil at the White House. Frank DeAngelis, principal of Colorado's Columbine High School, where a massacre in 1999 killed 15 people, said that "these senseless deaths" have to stop.

Panicked parents had raced to the Sandy Hook 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, New Jersey; Pete Yost in Washington; Michael Melia in Hartford; and the AP News Research Center in New York.