Story highlights Officials: Lanza killed 26 people at a school and his mother before he was found dead

Suspect's aunt: Lanza was "never in trouble with the law, never in trouble with anything"

"You could definitely tell he was a genius," a neighbor and former classmate says

Another former classmate recalls him as "just a kid" who didn't show violent tendencies

He was a 20-year-old man who, by multiple accounts, was incredibly smart and quiet. He didn't appear to have any run-ins with the law.

But Adam Lanza's seemingly innocuous life was thrust into the spotlight when authorities said he perpetrated the second deadliest school shooting in U.S. history.

On Friday morning, Lanza walked into Sandy Hook Elementary School in Newtown, Connecticut, dressed in black battle fatigues and a military vest" and began firing, according to a law enforcement source. By the time he was done, 26 were dead -- 20 of them young students.

The bloodshed ended when Lanza's own life did. He was found dead in a classroom with three firearms: a .223-caliber Bushmaster rifle and two pistols, a Glock and a Sig Sauer.

Before Friday's rampage, authorities said, Lanza killed his mother in their home in Newtown's Sandy Hook community.

A yearbook photo of Adam Lanza, taken during his sophomore year in 2008.

The mass tragedy left people in the Connecticut town, around the country, around the world asking questions. Who was this man, and why did he do this?

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 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 his 24-year-old brother Ryan.

Authorities have offered few details about 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," but he 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," the aunt, who lives in Crystal Lake, Illinois, said. "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."

Others in Newtown who knew Lanza said they had no idea he'd ever be responsible for such horror.

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 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 any link between autism and violence.

"Some public comments have drawn potentially inaccurate and stigmatizing conclusions about a link between the diagnosis and a propensity for violence and lack of empathy," according to a statement from the Autism Research Institute's Autistic Global Initiative Project.

"Autism is not a mental health disorder - it is a neurodevelopmental disorder. ... 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, not anti-social, 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."

The suspect'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," he 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."