State police said students and faculty were killed during the shooting at Sandy Hook Elementary School in Newtown.
Family and friends remember Adam Lanza as many things — intelligent, nerdy, goth, remote, thin.
Now the world will always remember him as a mass murderer. The 20-year-old man is believed to have killed his mother, pictured at right in a photo obtained by NBC News, before he gunned down more than two dozen people, 20 of them children, at a Connecticut grade school and committed suicide.
He might have suffered from a personality disorder, law enforcement officials said.
So far, authorities have not spoken publicly of any possible motive, although state police said Saturday that they had uncovered "very good evidence" as to "how, and more importantly why, this occurred."
Lanza attended Newtown High School, and news clippings from recent years show him on the honor roll. Joshua Milas, a classmate who was in the technology club with Lanza, said that he 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," said Milas, who graduated in 2009. "He was probably one of the smartest kids I know. He was probably a genius."
The tech club held "LAN parties" — short for local area network — in which students would gather at a member's home, hook up their computers into a small network and play games. Gloria Milas, Joshua's mother, said Adam Lanza's mother hosted one of the parties.
She recalled a school meeting in 2008 organized by the gunman's mother to try to save the job of the club's adviser. At the meeting, Milas said, Adam Lanza's brother Ryan said a few words in support of the adviser, who he said had taken his brother under his wing.
"My brother has always been a nerd," Ryan Lanza said then, according to Milas. "He still wears a pocket protector."
Olivia DeVivo, now a student at the University of Connecticut, recalled that Lanza always came to school toting a briefcase and wearing his shirt buttoned all the way up. "He was very different and very shy and didn't make an effort to interact with anybody" in his 10th-grade English class, she said.
"You had yourself a very scared young boy who was very nervous around people," said Richard Novia, who was the school district's head of security and adviser to the high school's Tech Club, of which Lanza was a member. He added: "He was a loner."
Novia said Lanza had extreme difficulties relating to fellow students and teachers, as well as a strange bodily condition: "If that boy would've burned himself, he would not have known it or felt it physically."
Lanza would also go through crises that would require his mother to come to school to deal with. Such episodes might involve "total withdrawal from whatever he was supposed to be doing, be it a class, be it sitting and read a book," Novia said.
When people approached Lanza in the hallways, he would press himself against the wall or walk in a different direction, clutching his black case "like an 8-year-old who refuses to give up his teddy bear," said Novia, who now lives in Tennessee.
Authorities say Adam Lanza shot his mother at their home before driving her car to Sandy Hook Elementary School and — armed with at least two handguns — carried out the massacre, officials said.
Lanza's family was struggling to make sense of what happened and "trying to find whatever answers we can," his father, Peter Lanza, said in a statement late Saturday that also expressed sympathy for the victims' families.
Investigators were trying to learn as much as possible about Lanza and questioned his brother, who is not believed to have any involvement in the rampage.
Lanza's parents filed for divorce in 2008, according to court records. His father, Peter Lanza, lives in Stamford, Conn., according to public records, and he reportedly works as a tax director for General Electric.
Nancy Lanza's mother was too distraught to speak when reached by phone at her home in Brooksville, Fla.
"I just don't know, and I can't make a comment right now," Dorothy Hanson, 78, said in a shaky voice as she started to cry. She said she hadn't heard anything official about her daughter and grandsons.
She declined to comment further and hung up.