Lanza allegedly murdered his mother, Nancy, at their home and then took her car and some of her guns to Sandy Hook Elementary School, where he broke in and opened fire, killing 26 students and staffers.
A Connecticut official said the mother, a gun enthusiast who practiced at shooting ranges, was found dead in her pajamas in bed, shot four times in the head with a .22-caliber rifle.
Lanza was wearing all black, with an olive-drab utility vest with lots of pockets, during the attack.
As investigators worked to figure out what drove him to lash out with such fury - and why he singled out the school - federal agents said that he had fired guns at shooting ranges over the past several years but that there was no evidence he did so recently as practice for the rampage.
Debora Seifert, a spokeswoman for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said both Lanza and his mother fired at shooting ranges, and also visited ranges together.
"We do not have any indication at this time that the shooter engaged in shooting activities in the past six months," Seifert said.
Investigators have found no letters or diaries that could explain the attack.
Of the few clues we know from inside the Lanza home, it's that his mother loved guns and passed that passion on to Adam.
And yet when a former classmate heard that the two went to firing ranges together, Allen Diaz found that strange.
"My initial response was I never really imagined Adam wanting to hold a gun. Because in my mind I don't imagine quiet shy people going to a shooting range," Diaz said.
"The passion here is so obviously damaging to the son," said Stephen Reich, Ph.D., Forensic Psychology Group.
Stephen Reich is a Forensic Psychologist. He questions why given Adam's personality struggles, Nancy Lanza would expose her son repeatedly to guns.
"In the context of a long-standing personality disorder, long-standing, going back 15 years, why is an area chosen which is violent and distancing," Reich said.
By all accounts, Nancy Lanza was a devoted mother. One psychologist told Eyewitness News she may have used her interest in guns to connect with a disconnected Adam and draw him out.
Still there's no mistaking the deadly power of military-style semi-automatic rifles, like the Bushmaster, owned by Nancy Lanza but used by her son in his rampage.
Years ago, in an Eyewitness News investigation, we easily and legally obtained one with no background check.
First, by ordering it on the internet and having it delivered by mail.
Another time, through a private purchase picking it up at the owner's home, no identification required.
"I don't believe a weapon like this should be easy to obtain. I believe that these weapons should be very limited, there use limited," said Lt. Frank Bisceglia, Harrison Police Department.
Lanza is believed to have used a Bushmaster AR-15-style rifle, a civilian version of the military's M-16. It is similar to the weapon used in a recent shopping mall shooting in Oregon and other deadly attacks around the U.S. Versions of the AR-15 were outlawed in this country under the 1994 assault weapons ban, but the law expired in 2004.
Investigators are now inspecting every aspect of his life for a possible motive to the rampage, but one possible clue was badly damaged.
ABC News has learned a computer at the Lanza home was smashed, possibly to hide evidence.
Friends said Adam Lanza stared down at the floor and didn't speak when his mother brought him around. They knew he'd switched schools more than once and that she'd tried home schooling him. But while she occasionally expressed concern about his future during evenings at the bar, she never complained about anything at all.
California resident Ryan Kraft told KCAL-TV in Los Angeles that when he was a teenager he lived a few doors down from the Lanza family and used to babysit Adam Lanza, then 9 or 10 years old. He said the boy "struck me as an introverted kid."
"His mom Nancy had always instructed me to keep an eye on him at all times, never turn my back or even go to the bathroom or anything like that," he said. "Which I found odd, but I really didn't ask. It wasn't any of my business. But looking back at it now, I guess there was something else going on."
Despite the challenges, the trappings of Lanza's life in Newtown were comfortable. When she and then-husband Peter Lanza moved to the central Connecticut community in 1998 from southern New Hampshire, they bought a brand new 3,100-square-foot colonial set on more than two acres in the Bennett's Farm neighborhood. Nancy Lanza had previously worked as a stock broker at John Hancock in Boston and her husband was a successful executive.
When the couple divorced in 2009, he left their spacious home to Nancy Lanza and told her she would never have to work another day in her life, said Marsha Lanza of Crystal Lake, Ill., Lanza's aunt. The split-up was not acrimonious and Adam spent time with both his mother and father, she said.
Those who knew Nancy Lanza recall her as very generous, often giving money to those she met and doing volunteer work.
When a mutual friend sought a loan from an acquaintance, Jim Leff, and Leff asked for collateral, Lanza intervened.
"Nancy overheard the discussion, and, unblinkingly, told him she'd just write him a check then and there," Leff recalled on his blog in a post after Lanza's death. "While I'm far from the most generous guy in the world, it's not often that I feel stingy. But I learned something from that. I should have just written him the check. She was right."
Mark Tambascio recalled the time Lanza invited him and his brother to attend a Boston Red Sox game, buying them tickets atop the outfield wall known as the Green Monster, and refusing any talk of repayment.
There were moments when she appeared carefree. Inside My Place on Sunday, friends passed around a book of photos from a 2008 sailing trip off Newport, R.I., including one showing Lanza, her eyes gently closed and head tilted back as the sea breeze blew through her hair. "Dreamer!" read the caption.
Neighbors knew her from the monthly gathering of women who rotated between homes for games of the dice game bunko. Lanza enthused about gardening, while poking fun of the fact that few could see the result because her house was set back from the road on a low rise, partly cloaked by trees.
"She used to give me a hard time, you know, because I put out all these Christmas lights, and she said, 'I put out mine, too, but you can't even see them,'" said Rhonda Cullens, who lives one street over.
Lanza also began telling friends that she'd bought guns and had taken up target shooting, John Tambascio said.
Marsha Lanza told the Chicago Sun-Times that Nancy Lanza wanted guns for protection. "She prepared for the worst," Marsha Lanza told the newspaper. "I didn't know that they (the guns) would be used on her."
Guns were her hobby," Dan Holmes, who got to know Lanza while doing landscaping work for her, told The Washington Post. "She told me she liked the single-mindedness of shooting."
But while trips to shooting ranges gave Lanza an outlet, she returned home to the ever-present challenges of raising a son with intractable problems.
At Newtown High School, Adam Lanza was often having crises that only his mother could defuse.
"He would have an episode, and she'd have to return or come to the high school and deal with it," said Richard Novia, the school district's head of security until 2008, who got to know the family because both Lanza sons joined the school technology club he chartered.
Novia said Adam Lanza would sometimes withdraw completely "from whatever he was supposed to be doing," whether it was sitting in class or reading a book.
Adam Lanza "could take flight, which I think was the big issue, and it wasn't a rebellious or defiant thing," Novia said. "It was withdrawal."
The club gave the boy a place where he could be more at ease and indulge his interest in computers. His anxieties appeared to ease somewhat, but they never disappeared. When people approached him in the hallways, he would press himself against the wall or walk in a different direction, clutching tight to his black briefcase.
Marsha Lanza described Nancy Lanza as a good mother.
"If he had needed consulting, she would have gotten it," Marsha Lanza said. "Nancy wasn't one to deny reality."
But friends and neighbors said Lanza never spoke about the difficulties of raising her son. Mostly she noted how smart he was and that she hoped, even with his problems, that he'd find a way to succeed.
"We never talked about the family," John Tambascio said. "She just came in to have a great time."
Nancy Lanza told a divorce mediator in 2009 that she didn't like to leave Adam alone and that she would care for him as long as he needed it.
When Lanza's parents divorced in 2009, the settlement left his mother with a comfortable income and the comfort of knowing that the then-17-year-old boy would have his education paid for and his medical insurance covered.
If there was bitterness and anger between Nancy and Peter Lanza, it is not described in court papers. And there was no mention of any lingering mental health or medical issues for Adam Lanza, nothing that could even hint at the horror he would unleash three years later.
In working through the terms of their divorce, the couple spent considerable time talking about how to provide for Adam Lanza's well-being, said Paula Levy, a mediator who worked with the couple.
During their meetings, the couple mentioned that Adam Lanza had been diagnosed with Asperger's syndrome, an autism-like disorder, Levy said. But the Lanzas were in complete agreement on how to address Adam's needs and said little about the details of his condition, Levy said.
"The only two things I remember them saying is that she really didn't like to leave him alone and I know they went out of their way to accommodate him," said Levy, who recalled Nancy and Peter Lanza as very respectful of each other and equally concerned about their son's needs.
"They worked together about it," Levy said. "The mom, Nancy, pretty much said she was going to take care of him (Adam) and be there as much as he needed her, even long-term."
While she would not disclose details of their discussions, Levy wanted to make clear that the Lanzas were loving parents who wanted the best for their son.
"These people are soft-spoken, gentle, both of them saying, 'What can we do to help him?'" Levy said.
The lawyer who represented Nancy Lanza in the divorce also spoke positively of her, calling her courteous and polite.
"She was an intelligent woman who we were pleased to represent," said the firm of Piazza, Simmons & Grant in Stamford.
The Lanzas married in June 1981 in Kingston, N.H. Nancy Lanza filed for divorce in 2009, by then living with her son in the home where she was found dead in her pajamas, on her bed.
The documents suggest little argument. The couple agreed to split up their jewelry, clothes and family photos. Adam would live with his mother, the couple agreed, and they agreed to talk about the important decisions.
If it turned out they couldn't agree on something related to Adam's upbringing, Nancy Lanza "shall make the final decision," according to the Sept. 24, 2009, settlement approved by Judge Stanley Novak.
There is nothing in the divorce court file that discusses the relationship's underlying problems. The file simply says the marriage "has broken down irretrievably and there is no possibility of getting back together."
Nancy Lanza, received $289,800 in alimony this year. It was to continue until December 2023, with slight increases each year for cost of living.
As part of the divorce, both Nancy and Peter Lanza were ordered to attend a parenting education program, standard practice in Connecticut. The provider, Family Centers Inc., certified that both completed the required sessions.
Authorities pored over computer, cellphone and credit card records trying to piece together the Lanza family's days leading up to the shooting. Peter Lanza, in a statement this weekend, said that like everyone else, he could not comprehend what had unfolded.
"We too are asking why," he said. "We have cooperated fully with law enforcement and will continue to do so. Like so many of you, we are saddened, but struggling to make sense of what has transpired."
Asperger's is a mild form of autism often characterized by social awkwardness. While people with the disorder can become frustrated more easily, there is no evidence of a link between Asperger's and violent behavior, experts say.
Associated Press writer Matt Apuzzo in Southbury, Conn. and Michael Tarm in Crystal Lake, Ill. contributed to this report.
(The Associated Press and ABC News contributed to this report)
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