A law enforcement official speaking on condition of anonymity said investigators believe Lanza attended the school several years ago but appeared to have no recent connection to the place.
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 law enforcement official said investigators were unable to establish any connection so far between her and the school.
Adam Lanza's older brother, now 24 and living in Hoboken, N.J., was being questioned, a law enforcement official said. He told authorities that his brother was believed to suffer from a personality disorder, the official said, speaking on condition of anonymity because he was not authorized to speak on the record about the unfolding investigation.
The official did not elaborate, and it was unclear exactly what type of disorder he might have had.
Ryan Lanza had been extremely cooperative and was not under arrest or in custody, but investigators were still searching his computers and phone records. Ryan Lanza told law enforcement he had not been in touch with his brother since about 2010.
Brett Wilshe, a friend of Ryan Lanza's, said he sent him a Facebook message Friday asking what was going on and if he was OK. According to Wilshe, Lanza's reply was something along the lines of: "It was my brother. I think my mother is dead. Oh my God."
Adam Lanza attended Newtown High School, and several local news clippings from recent years mention his name among the school's honor roll students.
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.
An official who spoke on condition of anonymity said it was not clear that Adam Lanza had a job, and there was no indication of law enforcement interviews or search warrants at a place of business.
A neighbor in Newtown, Rhonda Cullens, said she knew Nancy Lanza from monthly get-togethers the neighborhood women had a few years back for games of bunco, a dice game.
"She was a very nice lady," Cullens said. "She was just like all the rest of us in the neighborhood, just a regular person."
Cullens recalled that Lanza liked to garden and to make her house look nice for the holidays. Lanza joked, though, that no one noticed because the house was out of view, up a hill, she said.
Sandeep Kapur, who lives two doors down from the Lanza family in Newtown, said he did not know them and was unaware of any disturbances at the Lanza house in the three years that he and his family have been in the neighborhood.
He described the area as a subdivision of well-tended, 15-year-old homes on lots of an acre or more, where many people work at companies like General Electric, Pepsi and IBM. Some are doctors, and his next-door neighbor is a bank CEO, said Kapur, a project manager at an information technology firm.
"The neighborhood's great. We have young kids, and they have lots of friends," he said. "If you drive past this neighborhood, it gives you a really warm feeling."
Keyser reported from Chicago. Contributing to this report were Associated Press writers Adam Geller and Matt Apuzzo in Newtown, Conn.; Dave Collins in Hartford, Conn.; Michael Tarm in Crystal Lake, Ill.; and Michael Rubinkam.