Last Updated 4:58 p.m. ET
The massacre of 26 children and adults at a Connecticut elementary school elicited horror and soul-searching around the world even as it raised more basic questions about why the gunman, a 20-year-old described as brilliant but remote, was driven to such a crime and how he chose his victims.
Investigators were trying to learn more about Adam Lanza and questioned his older brother, who was not believed to have been involved in the rampage at Sandy Hook Elementary.
Police shed no light on the motive for the second-deadliest school shooting in U.S. history.
A law enforcement source told CBS News that Adam Lanza had three weapons with him during the attack: Two handguns (a Sig Sauer and a Glock), and a Bushmaster .223 assault rifle; spent shells were found in the school. Those three weapons were registered to his mother.
There was also a fourth weapon (a long gun) found in the car he drove to the school.
In addition, the source told CBS News there were "a few" other weapons found at the mother's house. The registry of the weapon in the car and at the house are still be checked. The tracing of all the weapons - such as where and when they were purchased - is still being conducted.
The Associated Press reports a Henry repeating rifle, an Enfield rifle and a shotgun were recovered. It was not clear exactly where those weapons were found.
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's aunt said her nephew was raised by kind, nurturing parents who would not have hesitated to seek mental help for him if he needed it.
Marsha Lanza, of Crystal Lake, Ill., said she was close with Adam Lanza's mother, Nancy, and sent her a message on Facebook on Friday morning asking how she was doing. Nancy Lanza never responded.
Marsha Lanza described Nancy Lanza as a good mother and kind-hearted.
If her son had needed counseling, "Nancy wasn't one to deny reality," she told The Associated Press late Friday.
Marsha Lanza said her husband saw Adam as recently as June and recalled nothing out of the ordinary about him.
A nurse who had been with the school for 15 years, told CBS News' Scott Pelley that no one knows Nancy Lanza and that she was not a staff teacher, after earlier reports said Nancy Lanza worked at the school. The nurse said its conceivable that Nancy Lanza was a subsitute that she never met, but very unlikely. No other staff member the nurse has talked to has ever heard of Nancy Lanza. The nurse also knows of no connection between the shooter and the school.
Asked at a news conference whether Lanza had left any emails or other writings that might explain the rampage, 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.
Officals also said that the suspect forced his way into the school; he was not let in voluntarily.
The tragedy plunged the picturesque New England town of 27,000 people into mourning.
"People in my neighborhood are feeling guilty about it being Christmas. They are taking down decorations," said Jeannie Pasacreta, a psychologist who volunteered her services and was advising parents struggling with how to talk to their children.
Lanza shot his mother, Nancy Lanza, drove to the school in her car with at least three of her guns, and opened fire in 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, duck under their desks or hide in closets as shots reverberated 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.
Maryann Jacob, a clerk in the school library, was in there with 18 fourth-graders when they heard a commotion and gunfire outside the room. She had the youngsters 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 it was the police.
"One of them slid his badge under the door, and they called and said, `It's OK, it's the police,"' she said.
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. It was not clear whether he held a job.
At least one parent said Lanza's mother was a substitute teacher at the school. 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, and investigators searched his computers and phone records, but he told law enforcement he had not been in touch with his brother since about 2010.
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 for autopsies.
The gunman forced his way into the kindergarten-through-fourth-grade school, authorities said. He took three guns into the school -- a Glock and a Sig Sauer, both semiautomatic pistols, and a .223-caliber Bushmaster rifle, according to an official who was not authorized to discuss information with reporters and spoke on the condition of anonymity. The weapons were registered to his slain mother.
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.
His 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 GE.
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. "He just said he was very thin, very remote and was one of the goths," she said.
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 that 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."
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 and beyond.
"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 parliamentary elections. In China, which has seen several knife rampages at schools in recent years, the attack quickly consumed public discussion.
In Newtown, 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.
"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."