(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But those who knew the shooter struggled to reconcile the difference between the quiet, withdrawn 20-year-old without a criminal record and the man who wore black fatigues and a military vest and brought death to Sandy Hook Elementary School Friday.
Police say the shooter, Adam Lanza, killed his mother, Nancy, in their home before walking into the school and fatally shooting 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 whether they know a motive for the attack, which has left those who knew Lanza trying to discern whether anything in his past could have foreshadowed the present.
"He was a shy kid, quiet," said Alan Diaz who knew Lanza several years ago. "He would sometimes stand in the corner. We knew he was socially awkward and we just accepted it. But he was never threatening."
Diaz told CNN that he tried many times to to talk to Lanza to "try to include him."
Candles burn next to a lighted tree at a makeshift shrine in Newtown, Connecticut, commemorating the victims of the mass shooting at Sandy Hook Elementary School on December 14, 2012.
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Connecticut State Police officers search outside St. Rose of Lima Roman Catholic Church in Newtown, Connecticut, on Sunday, December 16, after a threat prompted authorities to evacuate the building. Investigators found nothing to substantiate the reported threat, a police official said, declining to provide additional details. The church held Sunday services following last week's mass shooting at Sandy Hook Elementary School in Newtown.
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Lanza moved to Connecticut from Kingston, New Hampshire, with his parents and older brother Ryan, according to a booklet for 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 in the young man's life after that isn't clear, except that he continued to live in Newtown.
His father, Peter Lanza, remarried and moved to an area not far from the town, an official said.
Peter Lanza was questioned by authorities after the shooting, as was Adam Lanza'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 had to home-school her son.
"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."
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A 'genius'
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."
Alan Diaz said he was in the Technology Club with Lanza at Newtown High School. The group broadcast sports and other school events at school, and worked on computers.
Because Diaz was one of the few students at school who talked to Lanza, Diaz said, he thought Adam probably considered him a friend.
"Eventually he started opening up more to us on the tech team, telling us jokes even," Diaz recalled.
"I never would have thought he was capable of something like this," he said. "I once wondered that if he became isolated he might hurt himself, but never another person."
Diaz recalled that Lanza loved to wear khakis with a belt and green plaid shirt.
"He had a briefcase, really a laptop bag all the time," Diaz recalled. "He was smart. I would call him a genius."
Lanza took honors classes and once taught other students how to build a computer.
"He was really into it and very good at that class," Diaz said.
Diaz ran into Nancy Lanza, Adam's mother, not too long ago, he said.
"When I talked to Nancy that time, about how he was doing, she said he's been going to the (gun) range a lot recently," Diaz said. "That he'd taken that up as a hobby."
As a 13-year-old, Lanza would occasionally ride the bus to school, often sitting in the back, 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.
A 'polite' mother
"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 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."
CNN's Susan Candiotti, Mary Snow and Lisa Desjardins reported from Newtown, Connecticut. CNN's Tim Lister, Terry Frieden, John King and Greg Botelho also contributed to this report.
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Erica Simmons rings the campus bell at Agnes Scott College in Decatur, Georgia, during a nationwide commemoration December 21, marking a week since the Newtown, Connecticut, mass shooting. Church bells rang out across the country at 9:30 ET Friday to remember those who died in the gun rampage.
A woman pauses at a streetside memorial during a moment of silence on December 21 in Newtown.
Connecticut State Police block the road to Sandy Hook Elementary School during a moment of silence on December 21. A week ago, a gunman forced his way into the school and shot and killed 26 people, including 20 children.
A woman bows her head in Newtown's Sandy Hook village on December 21.
People observe a moment of silence for the school shooting victims at the Blue Colony Diner in Newtown on December 21.
Children in Newtown, excluding Sandy Hook Elementary, return to classes on Tuesday, December 18, four days after the shooting at the elementary school.
U.S. Rep. David Cicilline, D-Rhode Island, holds a news conference with the Brady Campaign to discuss gun violence. In attendance with the Brady Campaign were several survivors of gun violence and family members of victims of gun violence.
Members of the human rights group Volunteers Against Crime and Corruption light candles showing the names of those killed during the shooting at Sandy Hook Elementary School, during a prayer vigil in front of the U.S. Embassy in Manila, Philippines on December 18.
Children light candles to pay their respects to the victims of the Newtown, Connecticut, shooting at the main square in Tirana, Albania, on Monday, December 17. The deadly gun rampage at Sandy Hook Elementary School has provoked strong reactions from around the world.
Protesters march on the National Rifle Association's Capitol Hill lobbyist offices in Washington on December 17.
New York Mayor Michael Bloomberg speaks out for stronger gun control at a press conference at City Hall on December 17. Bloomberg, co-chair of Mayors Against Illegal Guns, was joined by victims and survivors of gun violence.
Chris Foye, whose son Chris Owens was killed by a stray bullet in 2009, stands with other survivors and family members of gun violence at Bloomberg's press conference on December 17 in New York.
People pay their respects on December 17 at a makeshift shrine in Newtown to the victims of Friday's elementary school shooting. Funerals began Monday in the Connecticut town.
Traders on the floor of the New York Stock Exchange hold a moment of silence on December 17 in honor of the shooting victims.
Young people mourn at Newtown High School before a memorial service attended by President Obama on Sunday, December 16.
President Barack Obama waits to speak at an interfaith vigil for the shooting victims from Sandy Hook Elementary School December 16 at Newtown High School.
Mourners comfort one another December 16 before U.S. President Barack Obama speaks at an interfaith vigil for the shooting victims.
Two women embrace before the interfaith vigil at Newtown High School on Sunday evening.
From left: Newtown residents Claire Swanson, Kate Suba, Jaden Albrecht, Simran Chand and New London, Connecticut, residents Rachel Pullen and her son, Landon DeCecco, hold candles at a memorial for victims on Sunday, December 16, in Newtown, Connecticut.
A young boy walks past Christmas trees set up at a makeshift shrine to the shooting victims in Newtown, Connecticut, on December 16.
Ty Diaz is kissed by his mother, Yvette, at a memorial down the street from Sandy Hook on December 16.
Teddy bears, flowers and candles in memory of those killed are left at a memorial down the street from the school on December 16.
Two teenagers embrace at a makeshift shrine to the victims in Newtown on December 16.
Nuns pay their respects at a makeshift shrine to the victims on December 16.
Members of Sisters of Christian Charity go to lay flowers in front of the Sandy Hook Elementary School on December 16 in Newtown.
A woman receives a hug as she leaves morning service December 16 at Trinity Church in Newtown near the elementary school.
Parishioners pay their respects to the victims of the elementary school shooting while attending Mass at St. Rose of Lima Roman Catholic Church in Newtown on December 16.
Police officers honor the victims of the school shooting at the St. Rose of Lima Roman Catholic Church on December 16 in Newtown.
The Jacksonville Jaguars have a moment of silence in honor of the Sandy Hook Elementary School shooting victims before their game against the Miami Dolphins on December 16.
A parishioner kneels in front of a makeshift memorial at St. Rose of Lima Roman Catholic Church in Newtown on December 16.
A police officer removes flowers from a busy intersection on December 16 in Newtown. Police said they were afraid the memorial, left for the victims of the Sandy Hook Elementary School shooting, would cause a traffic hazard.
A woman hugs her daughter on the steps of Trinity Church on December 16 in Newtown.
Angel wood cutouts for each of the 27 victims are set up on hillside in Newtown on December 16.
People in Bangalore, India, hold cards and photographs of the slain at a candlelight vigil outside a Catholic church on December 16.
J.J. Watt of the Houston Texans shows his glove in remembrance of the victims before the start of a game against the Indianapolis Colts on December 16 in Houston.
Donna Soto, right, mother of Victoria Soto, the first-grade teacher at Sandy Hook Elementary School who was shot and killed while protecting her students, hugs her daughter Karly while mourning their loss at a candlelight memorial at Stratford High School on Saturday, December 15, in Stratford, Connecticut.
Firefighters kneel to pay their respects at a makeshift memorial near the school in Newtown on Saturday.
A child lights a candle at a memorial filled with flowers, stuffed toys and candles outside of Saint Rose of Lima Church near Sandy Hook Elementary School in Newtown, Connecticut on Saturday.
Candles light up a memorial outside of Saint Rose of Lima Church in Newtown.
Lucas, Kelly and Michael DaSilva pray and embrace at a makeshift memorial near the school in Newtown.
People are overcome with emotion Saturday at a makeshift memorial near Sandy Hook Elementary School in Newtown.
Residents arrive Saturday to pay tribute to the victims of an elementary school shooting in Newtown, Connecticut.
A couple carry balloons to place at a curbside shrine to in Newtown on Saturday.
A mother and daughter attend a prayer service at St. John's Episcopal Church in Newtown on Saturday.
Flowers and signs of sympathy adorn the street leading to Sandy Hook Elementary School.
Claudia Urbiana and daughter Jocelyne Cardenas, left, hug outside of the entrance to the Sandy Hook school.
A makeshift memorial with flowers, stuffed toys and candles sit outside Saint Rose of Lima Roman Catholic Church in Newtown, Connecticut, on Saturday.
A man bows his head as he stands at a makeshift memorial, outside Saint Rose of Lima Roman Catholic Church in Newtown on Saturday, December 15.
New Jersey resident Steve Wruble, who was moved to drive out to Connecticut to support local residents, grieves for victims at the entrance to Sandy Hook village in Newtown on Saturday.
People attend a prayer service in Newtown on Saturday to reflect.
A mother hugs her children after paying tribute to the victims in Newtown on Saturday, December 15.
Newtown High School student Trevor Lopez stands outside of a church where residents have come to pray and reflect on Saturday
Andrea Jaeger places flowers and a candle at a makeshift memorial outside a firehouse near Sandy Hook Elementary School on Saturday.
The U.S. flag flies at half-staff above the White House on Saturday.
Ken Kowalsky and his daughter Rebecca, 13, embrace while standing at the end of the road leading to Sandy Hook Elementary School on Saturday.
A woman puts a flower near crosses planted by Rio de Paz (Rio of Peace), in memory of the victims of the Sandy Hook Elementary school shooting on Copacabana beach in Rio de Janeiro on Saturday.
A woman sits during a service at the Asylum Hill Congregational Church in Hartford, Connecticut on Friday, December 14.
Fans at the NBA game between the Utah Jazz and the Phoenix Suns participate in a moment of silence for the victims of the Newtown shooting on Friday in Phoenix.
An overflow crowd listens to a church service held at the St. Rose of Lima Catholic Church in Newtown on December 14.
People gather for a prayer vigil at St. Rose Church on Friday.
People gather for a vigil outside the White House in Washington following the Connecticut elementary school shooting on Friday.
Candles burn as people gather for a vigil outside the White House.
Hartford, Connecticut, Mayor Padro Segarra speaks emotionally about the students and teachers who died earlier in the day at Sandy Hook Elementary School in nearby Newtown at a candlelight vigil at Bushnell Park in Hartford on Friday.
Cynthia Alvarez is comforted by her mother, Lilia, as people gather for a prayer vigil at St. Rose of Lima Roman Catholic Church in Newtown.
People gather for a prayer vigil at St. Rose Church in Newtown.
People gather for a prayer vigil at St. Rose Church.
People gather inside the St. Rose Church to remember the shooting victims on Friday
A woman bows her head during a vigil for the shooting victims at St. Rose Church.
People gather in the St. Rose Church for a memorial service Friday.
Women comfort each other during the vigil at St. Rose Church.
A woman looks on during the vigil at St. Rose Church.
People hug outside of the Newtown United Methodist Church on Friday, near the site of the shootings at the Sandy Hook school.
A flag at the U.S. Capitol flies at half-staff after President Barack Obama ordered the action while speaking from the White House. Obama called for "meaningful action" in the wake of the school shooting.
Julie Henson of San Francisco joins other people outside the White House to participate in a candlelight vigil on Friday.
Obama wipes tears as he makes a statement in response to the shooting on Friday.
Washington resident Rachel Perrone, left, and her 5-year-old son, Joe, center, join others outside the White House in a candlelight vigil.
Faisal Ali, right, of Colorado Springs, Colorado, joins the vigil outside the White House.
People gather outside the White House to participate in a candlelight vigil.
Supporters of gun control hold a candlelight vigil for victims of the shooting outside the White House.
Connecticut State Police officers walk out of St. Rose of Lima Roman Catholic Church after the Newtown church received a threat December 16.
Firefighters attach black bunting to a fire truck as a memorial at the fire station down the street from the Sandy Hook Elementary School in Newtown, Connecticut, on Saturday, December 15.
Connecticut Chief Medical Examiner H. Wayne Carver II talks to the media about the elementary school shooting during a press conference at Treadwell Memorial Park on December 15.
Zulma Sein is hugged by a family member outside of the entrance to the Sandy Hook School on Saturday.
Police officers keep guard at the entrance to the street leading to the Sandy Hook Elementary School on Saturday, December 15.
Connecticut State Police Lt. Paul Vance addresses the press on December 15.
Police officers stand at the entrance to the street leading to the Sandy Hook Elementary School on December 15.
Corinne McLaughlin, a student at the University of Hartford, bows her head during a candlelight vigil at Hartford, Connecticut's Bushnell Park on Friday, December 14, honoring the students and teachers who died at Sandy Hook Elementary School in nearby Newtown earlier in the day.
Distraught people leave the fire station after hearing news of their loved ones from officials on Friday.
Emergency workers stand in front of the Sandy Hook Elementary School in Newtown.
A child and her mother leave a staging area outside Sandy Hook Elementary School in Newtown, Connecticut, on December 14.
Members of the media converge on December 14 in front of an apartment at 1313 Grand Street in Hoboken, New Jersey. The apartment is believed to be connected to the Connecticut elementary school shooting.
Connecticut State Police spokesman Lt. J. Paul Vance, center, briefs the media on the elementary school shootings during a press conference at Treadwell Memorial Park on December 14 in Newtown.
People weep and embrace near Sandy Hook Elementary School on Friday, December 14.
A woman leans on a man as she weeps near Sandy Hook Elementary School on December 14.
President Barack Obama wipes a tear as he speaks about the shooting at Sandy Hook Elementary School during a press briefing at the White House on December 14.
A woman weeps near the site of a shooting at Sandy Hook Elementary School on December 14.
A woman weeps near Sandy Hook Elementary School on December 14.
People comfort each other near Sandy Hook Elementary School on December 14.
A man takes in the scene near Sandy Hook Elementary School on December 14.
A young girl is given a blanket after being evacuated from Sandy Hook Elementary School on December 14.
State police personnel lead children from the school.
Children wait outside Sandy Hook Elementary School in Newtown, Connecticut, after the shooting.
A boy weeps at Reed Intermediate School after getting news of the shooting at Sandy Hook Elementary School on December 14.
FBI SWAT team members walk along Dickinson Drive near Sandy Hook Elementary School on December 14.
An aerial view of Sandy Hook Elementary School in Newtown, Connecticut on December 14.
Connecticut State Troopers arrive on the scene outside Sandy Hook Elementary School on December 14.
A Connecticut State Police officer runs with a shotgun at the Sandy Hook Elementary School in Newtown on December 14.
Police patrol the streets around Sandy Hook Elementary School on December 14.
People try to deal with the shock of the attack outside Sandy Hook Elementary School on December 14.
Connecticut State Police secure the scene of the shooting on December 14.
People embrace outside Sandy Hook Elementary School on December 14.
A man escorts his son away from Sandy Hook Elementary School on December 14.
People take in the news outside Sandy Hook Elementary School on December 14.
People line up to enter Newtown Methodist Church near the the scene of the shooting on December 14.
A woman speaks with a Connecticut state trooper outside Sandy Hook Elementary School on December 14.
The streets around Sandy Hook Elementary are packed with first responders and other vehicles.
A view of the scene at Sandy Hook Elementary School after the shooting.
A young boy is comforted outside Sandy Hook Elementary School on December 14.
People embrace each other on December 14.
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