(CNN) -- Who was Adam Lanza?
He's a 20-year-old Connecticut man, but other than his age, the picture of him is blurry. But details of what law enforcement officials say he did Friday are all too vivid.
That morning, Adam 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 two firearms, a Glock and Sig Sauer. Another gun, a .223 Bushmaster, was found nearby in a car.
The massacre left people in the Connecticut community, around the country, around the world asking questions. Who was this man, and why did he do this?
A booklet for Newtown's Bennetts Farm neighborhood indicated that Adam Lanza moved to Connecticut from Kingston, New Hampshire, with his parents and older brother Ryan. He enjoyed soccer, skateboarding and video games, the booklet said.
In September 2009 -- when Adam 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.
Authorities hadn't divulged many details about Lanza as of Friday night. He 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.
His father, Peter Lanza, remarried and lived not too far from Newtown, an official said. He was questioned by authorities after the shooting. So, too, was 24-year-old brother Ryan Lanza, who was seen being taken away by police from a residence in Hoboken, New Jersey.
Yellow crime tape, meanwhile, surrounded the Newtown home that the gunman shared with his mother Nancy Lanza.
Law enforcement officers from various agencies streamed in and out. And around 3:15 p.m., an ambulance came to the scene and then left the house. A law enforcement officer has said that Nancy Lanza was found dead at the residence.
Neighbors -- who were ordered to evacuate after the shooting -- described their terror and fear, but many had little to say about the Lanza family.
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."
Others in Newtown who knew Adam Lanza said they had no inkling 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 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."
CNN's Susan Candiotti and Mary Snow reported from Newtown, Connecticut. CNN's Tim Lister, Terry Frieden and John King also contributed to this report. CNN's Greg Botelho wrote this story from Atlanta.