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."