"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.
But a national autism committee cautioned against speculating about a link between autism and violence.
"Autism is not a mental health disorder -- it is a neurodevelopmental disorder," said the Autism Research Institute's Autistic Global Initiative Project. "The eyes of the world are on this wrenching tragedy -- with 1 in 88 now diagnosed, misinformation could easily trigger increased prejudice and misunderstanding."
A former classmate told CNN affiliate WCBS that Lanza "was just a kid" -- not a troublemaker.
"I don't know who would do anything like this," the ex-classmate said, before walking away distraught. "This is unspeakable."
Lanza's father was also at a loss for explanation. He sent his condolences to the families of victims in a statement released Saturday.
"Our family is grieving along with all those who have been affected by this enormous tragedy," Peter Lanza said. "No words can truly express how heartbroken we are. We are in a state of disbelief and trying to find whatever answers we can."
In New Hampshire, relatives of Nancy Lanza released a statement in which they, too, expressed shock and sadness.
"On behalf of Nancy's mother and siblings we reach out to the community of Newtown and express our heartfelt sorrow for the incomprehensible and profound loss of innocence that has affected so many."