Adam Lanza was found dead in a classroom, and police recovered three weapons from the scene: a semiautomatic .223-caliber rifle made by Bushmaster and two handguns made by Glock and Sig Sauer, a source with knowledge of the investigation said.
Adam Lanza had no known criminal record, a law enforcement official said.
A member of Lanza's family told investigators that he had a form of autism, according to a law enforcement official who spoke under condition of anonymity due to the sensitive nature of the investigation.
Acquaintances struggled with fathoming the deadly actions being attributed to someone they had known.
Alex Israel was in the same class at Newtown High School with Adam Lanza, who lived a few houses down from her.
"You could definitely tell he was a genius," Israel told CNN, adding she hadn't talked with him since middle school. "He was really quiet, he kept to himself."
His former bus driver, Marsha Moskowitz, told CNN affiliate WABC that he was "a nice kid, very polite" like his brother.
"It's a shock to even know (the family)," she said. "You can't understand what happened."
A former classmate told CNN affiliate WCBS that Adam Lanza "was just a kid" -- not a troublemaker, not antisocial, not suggesting in any way that he could erupt like this.
"I don't know who would do anything like this," the classmate said, before walking away distraught. "This is unspeakable."