The tragedy plunged Newtown into mourning and added the picturesque New England community of handsome colonial homes, red-brick sidewalks and 27,000 people to the grim map of towns where mass shootings in recent years have periodically reignited the national debate over gun control but led to little change.
Signs around town read, "Hug a teacher today," "Please pray for Newtown" and "Love will get us through."
"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 struggling with how to talk to their children.
In the tightly knit town, nearly everyone seemed to know someone who died. Among them: well-liked Principal Dawn Hochsprung, who town officials say tried to stop the rampage and paid with her life; the school psychologist who probably would have helped survivors grapple with the tragedy; a teacher thrilled to have been hired this year; and a 6-year-old girl who had just moved to Newtown from Canada.
Authorities said Lanza had no criminal history; it was not clear whether he had a job. Lanza was believed to have suffered from a personality disorder, said a law enforcement official who spoke on condition of anonymity.
Another law enforcement official, also speaking on condition of anonymity, said Lanza had been diagnosed with Asperger's, a mild form of autism often characterized by social awkwardness. People with the disorder are often highly intelligent. While they can become frustrated more easily, there is no evidence of a link between Asperger's and violent behavior, experts say.
The law enforcement officials insisted on anonymity because they were not authorized to discuss the unfolding investigation.
Acquaintances describe the former honor student as smart but odd and remote.
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 district's head of security and adviser to the school's Tech Club, of which Lanza was a member. He added: "He was a loner."