In the aftermath of the horrific Sandy Hook Elementary School massacre, a shocked and grieving nation is grasping for answers to an agonizingly simple question: Who could do such a thing? Who could walk into a classroom full of 6-year-olds, innocent children with their entire lives ahead of them, and murder them? Police, neighbors, and relatives of 20-year-old suspect Adam Lanza, who reportedly killed himself with a single gunshot to the head after police officers rushed into the Newtown, Conn., school, have begun piecing together a portrait of his brief and troubled life. Here's what we know:

1. He had a web of psychiatric and emotional problems
One of the first details to emerge about Adam Lanza came from his brother, 24-year-old Ryan, who was briefly misidentified as the shooter. Ryan told ABC News that his brother "is autistic, or has Asperger's syndrome and a 'personality disorder.'" Former classmates described him as nervous, reported The New York Times, "with a flat affect." "This was a deeply disturbed kid," a family insider said. Adam reportedly had a disorder that prevented him from feeling pain. Autism and Asperger's are associated with difficulties in communicating, forming relationships, and feeling empathy for others, but not with premeditated violence, says Dr. Gabriella Rosen Kellerman at The Atlantic. Some types of "personality disorder," such as anti-social personality disorder, are associated with dangerous outbursts. 

2. Adam Lanza was extremely smart, and a loner
Relatives and former classmates of Adam agree that he was quite smart. "He really was a computer geek,'' said his aunt, Marsha Lanza of Chicago. "He was a very bright boy." "He was always different — keeping to himself, fidgeting, and very quiet," a classmate, Alex Israel, told The Times. Israel said Adam was "a supersmart kid, maybe just socially awkward, something just off about him." Neighbors described him as reclusive and troubled. "Adam Lanza has been a weird kid since we were 5 years old," said Tim Dalton, a neighbor and former classmate, on Twitter. "As horrible as this was, I can't say I am surprised."

3. He was devastated by his parents' divorce 
Four years ago, Adam's parents, Nancy and Peter Lanza, divorced after 28 years of marriage. "The breakup was traumatic, leaving the couple's sons devastated," report Harriet Alexander, David Barrett, Laura Donnelly, and Jon Swaine at Britain's The Telegraph. At the time, Ryan was away at college. Adam, four years younger, was living at home with his mother, who received $250,000 a year in alimony to support the household. Friends described Nancy as a friendly, social, and generous person, but said she also seemed "high-strung," according to The Times, "as if she were holding herself together." Her sister-in-law, Marsha Lanza, said she had "battled with the school district.... I'm not 100 percent certain if it was behavior, learning disabilities, I really don't know." Eventually, she took Adam out of the local high school and homeschooled him.

4. And he lived in a house full of guns
Nancy was Adam's first victim in the shooting spree. Police found her in bed, shot four times in the face. There were two guns in the room. The one that killed her, police say, belonged to her, as did the two powerful pistols and military-style semiautomatic rifle that Adam reportedly took to the school. Nancy's friends say she was a gun enthusiast who kept a small armory — two traditional hunting rifles, two handguns, and the semiautomatic rifle — for protection in case an economic meltdown resulted in chaos. She often took her sons to shooting ranges to make sure they knew how to handle guns, too. Despite all of the information that is coming out, however, even those closest to Adam say they could never have imagined such a tragedy happening. "Like so many of you, we are saddened, but struggling to make sense of what has transpired," Adam's father, Peter, said in a statement. "We too are asking why."

Sources: The Atlantic, CBS News, The Daily Beast, Daily Mail, The New York Times, Telegraph