Police have shed no light on a possible motive for the massacre in Newtown, how the gunman was driven to commit such a crime, and how he chose his victims. They say Lanza, who has not been officially named as the gunman, had no prior criminal background.
At this time, only state police sources, quoted by local media, have identified the gunman as Lanza, 20, and say when he was found dead in a hallway, he was carrying his brother Ryan Lanza's identification, which initially led to media reports that the 24-year-old was the suspect.
Lanza is reported to have begun his killing rampage Friday at his home, where officials say he shot his mother, before embarking on the massacre at the school.
"Our investigators at the crime scene and the secondary crime scene did produce some very good evidence in this investigation ... as to why this occurred," Connecticut State Police spokemsan Lt. Paul Vance said on Saturday when asked by reporters about a possible motive.
The Associated Press, citing official sources, says Lanza is believed to have shot mother in the face, before driving to the school in her car.
Lanza and his mother lived in a well-to-do part of prosperous Newtown, about 95 kilometres northeast of New York City.
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 General Electric.
Adam Lanza's aunt Marsha Lanza, of Crystal Lake, Ill., told The Associated Press that her nephew was raised by 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.
Vance has confirmed that the gunman had forced his way into the school. He was later found dead inside the building, apparently of a self-inflicted gunshot wound.
In the shooting, 18 children were pronounced dead at the school as well as six adults and the gunman. Two more children who were rushed to hospital were later pronounced dead.
Three guns were found — a Glock and a Sig Sauer, both pistols, inside the school, and a .223-calibre rifle in the back of the car. The guns were reportedly owned and registered to Nancy Lanza, Adam Lanza's mother.
The gunman was also wearing a bulletproof vest, ABC News reported and, according to CNN, was garbed in black battle fatigues and a military vest.
'Reserved' loner with 'some disabilities'
Richard Novia, the school district's head of security until 2008, told The Associated Press that Lanza clearly "had some disabilities." Novia had also served as adviser for the technology club at Newtown High School, where Lanza was a student.
He told The Associated Press that Lanza, who was a member of the club, did not seem to feel physical or emotional pain in the same way as his classmates.
"If that boy would've burned himself, he would not have known it or felt it physically," Novia said in a phone interview. "It was my job to pay close attention to that."
He also described "episodes" where Lanza seemed to withdraw "from whatever he was supposed to be doing," prompting his mother to come to the high school to deal with it.
Lanza "could take flight, which I think was the big issue, and it wasn't a rebellious or defiant thing," Novia said. "It was withdrawal."
The tech club was popular among socially awkward students. But Adam, while clearly smart, had problems that went beyond an adolescent lack of social skills, Novia said.
"You had yourself a very scared young boy, who was very nervous around people he could trust or he refused to speak with," Novia said.
Novia said that while other members of the club were acquainted with Lanza, none were close to him.
"Have you found his best friend? Have you found a friend?" Novia asked. "You're not going to. He was a loner."
Adam was not physically bullied, although he may have been teased, Novia said.
When people approached him in the hallways, he would press himself against the wall or walk in a different direction, clutching tight to his black case.
"The behaviour would be more like an 8-year-old who refuses to give up his teddy bear," Novia said. "What you knew with Adam is it was a possession. It was not a possession to be put at risk."
Even so, Novia said, his primary concern was that Adam might become a target for abuse by his fellow students, not that he might become a threat.
"Somewhere along in the last four years, there were significant changes that led to what has happened," Novia said. "I could never have foreseen him doing that."
Jim McDade, who lives a few houses from where Nancy Lanza was slain, said his family became acquainted with the two brothers and their mother because their children were about the same ages and rode the school bus together.
"There was certainly no indication of anything unusual that lets you think that a kid's going to do something like that," said McDade, who works in finance in New York. "There was nothing that would indicate anything going on behind the scenes that would lead to this horrible mess."
He recalled Adam Lanza as "a very bright kid."
Alex Israel, a former classmate of Adam Lanza's, told CNN he was a quiet loner and "reserved" and that "you could definitely tell he was a genius."
A law enforcement official speaking on condition of anonymity said Lanza appeared to have no recent connection to the school.
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 mother and father were divorced, but his father had remarried and lived not too far from Newtown, an official told CNN.
Police questioned Lanza's older brother, Ryan, of Hoboken, N.J., but he was not believed to have had any role in the rampage.
He told law enforcement he had not been in touch with his brother since about 2010.