(CNN) -- As with many murder-suicides, the gunman in the Newtown, Connecticut, shooting took to his grave the reasons that compelled him to kill more than two dozen people before taking his own life in the second-deadliest school shooting in U.S. history.
But those who knew the shooter struggled to reconcile the difference between the quiet, withdrawn 20-year-old without a criminal record and the man who donned black fatigues and a military vest and rained hell at Sandy Hook Elementary School last week.
Police said the shooter was Adam Lanza and that he killed his mother, Nancy, in their home before walking into the school and spraying with bullets 26 more people -- 20 of them children no older than 7.
The rampage ended when Lanza apparently took his own life in a classroom. With him were three firearms: a semiautomatic .223-caliber rifle made by Bushmaster and two handguns, a Glock and a Sig Sauer.
Connecticut law requires gun owners to be at least 21. The guns, authorities said, belonged to his mother.
Police have yet to disclose a motive for the attack -- which left those who knew Lanza trying to discern whether anything in his past could have foreshadowed the present.
Lanza moved to Connecticut from Kingston, New Hampshire, with his parents and older brother Ryan, according to a booklet for the town's Newtown's Bennetts Farm neighborhood. He enjoyed soccer, skateboarding and video games, the publication said.
In September 2009 -- when Lanza was 17 -- his mother and father divorced, court documents show.
What happened after that for him isn't clear, except that he continued to lived in the picturesque, 300-year-old Connecticut town.
His father, Peter, remarried and lived not far from Newtown, an official said. He was questioned by authorities after the shooting. So, too, was Adam's 24-year-old brother, Ryan.
Authorities have offered few details about Adam Lanza. He had no known criminal record, a law enforcement official said.
One of Lanza's aunts, Marsha, described him as a "quiet, nice kid" who had issues with learning, she said. Her husband is Lanza's paternal uncle.
"He was definitely the challenge of the family in that house. Every family has one," she told CNN affiliate WLS. "They have one. I have one. But never in trouble with the law, never in trouble with anything."
She said Lanza's mother "battled" with the school board and ended up having her son home-schooled.
"She had issues with school," said the aunt, who lives in Crystal Lake, Illinois. "I'm not 100% certain if it was behavior or learning disabilities, but he was a very, very bright boy. He was smart."
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."
As a 13-year-old, Adam Lanza would occasionally ride the bus to school, often sitting in the back and 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.
"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 Adam 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."
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