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.
Richard Novia, the school district's head of security until 2008, who also served as adviser for the school technology club, of which Lanza was a member, said he clearly "had some disabilities."
"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."
Amid the confusion and sorrow, stories of heroism emerged, including an account of Hochsprung, 47, and the school psychologist, Mary Sherlach, 56, rushing toward Lanza in an attempt to stop him. Both died.
There was also 27-year-old teacher Victoria Soto, whose name has been invoked as a portrait of selflessness and humanity among unfathomable evil. Investigators told relatives she was killed while shielding her first-graders from danger. She reportedly hid some students in a bathroom or closet, ensuring they were safe, a cousin, Jim Wiltsie, told ABC News.
"She put those children first. That's all she ever talked about," a friend, Andrea Crowell, told The Associated Press. "She wanted to do her best for them, to teach them something new every day."
There was also 6-year-old Emilie Parker, whose grieving father, Robbie, talked to reporters not long after police released the names of the victims but expressed no animosity, offering sympathy for Lanza's family.
"I can't imagine how hard this experience must be for you," he said.
On Saturday, Carver, the medical examiner, said that all the victims at the school were shot with a rifle, at least some of them up close, and that all were apparently shot more than once. All six adults killed at the school were women. Of the 20 children, eight were boys and 12 were girls.
Asked how many bullets were fired, Carver said, "I'm lucky if I can tell you how many I found."
Parents identified the children through photos to spare them some shock, Carver said.
Relatives of the shooter were at a loss for words.
"The whole family is traumatized by this event," said Donald Briggs Jr., police chief of Kingston, N.H., who knows the family. "We reach out to the community of Newtown and express our heartfelt sorrow for this incomprehensible and profound loss of innocence," the family said in a statement.
James Champion, Nancy Lanza's brother and a retired police captain in Kingston, N.H., said through the police chief that he had not seen his nephew in eight years. Champion, who still works as a part-time officer, said he would not discuss what might have triggered the rampage since the case is under 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.
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," said Novia, the tech club adviser.
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.
Even so, Novia said his main concern about Lanza was that he might become a target for teasing or abuse by other 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 Friday morning," Novia said. "I could never have foreseen him doing that."
Nancy Lanza, who was once a stockbroker for John Hancock in Boston and once lived in Kingston, N.H., was a kind, considerate and loving person, Briggs said.