Alex Israel was in the same class at Newtown High School with Lanza and lived a few houses down from him.
"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."
Alan Diaz said he was in the Technology Club with Lanza at Newtown High School. The group broadcast sports and other school events at school, and worked on computers.
Because Diaz was one of the few students at school who talked to Lanza, Diaz said, he thought Adam probably considered him a friend.
"Eventually he started opening up more to us on the tech team, telling us jokes even," Diaz recalled.
"I never would have thought he was capable of something like this," he said. "I once wondered that if he became isolated he might hurt himself, but never another person."
Diaz recalled that Lanza loved to wear khakis with a belt and green plaid shirt.
"He had a briefcase, really a laptop bag all the time," Diaz recalled. "He was smart. I would call him a genius."
Lanza took honors classes and once taught other students how to build a computer.
"He was really into it and very good at that class," Diaz said.
Diaz ran into Nancy Lanza, Adam's mother, not too long ago, he said.
"When I talked to Nancy that time, about how he was doing, she said he's been going to the (gun) range a lot recently," Diaz said. "That he'd taken that up as a hobby."
As a 13-year-old, Lanza would occasionally ride the bus to school, often sitting in the back, usually alone, said his former bus driver.
"He didn't sit with the other kids and didn't seem to have any friends," said Marsha Moskowitz, 52, who said she drove Lanza to school for three years.
"He was quiet, a very shy and reserved kid," she said, noting that Lanza was one of the older kids on the bus and did little to interact with the others. "No 13-year-old wants to ride the bus to school. It's kind of embarrassing for them."
The shooter's mother was also a quiet woman, said Moskowitz, though she admits she had limited interactions with her.
A 'polite' mother
"I didn't know (Nancy) as well as the other parents, but she was always very polite," said Moskowitz, who said she's been devastated by the news.
A relative told investigators that Lanza 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.