In his last few years, Adam Lanza shut himself off from the outside world almost completely, his troubles slowly escalating as his family splintered.
Adam Lanza's mother, Nancy.
In the summer of 2010, Mr. Lanza cut off contact with his father, Peter Lanza, a General Electric GE -0.29%executive whose marriage with his mother ended a year before, said a person with knowledge of family matters. It is unclear why Mr. Lanza refused to speak with his father, who made repeated attempts to contact him, this person said, but the breakdown in their relationship came as Peter Lanza started to get serious with his girlfriend, whom he married last year.
After Christmas 2010, Adam Lanza had no contact with his older brother, Ryan, who had moved away for a job in New York City, according to records and a person with knowledge of the family. And three years ago, high-school classmates recalled Adam Lanza suddenly disappearing midway through high school in Newtown, Conn. "He disappeared off the face of the Earth," according to one friend.
Then, last Friday morning, Adam Lanza fatally shot his final link to the outside world, his mother, Nancy Lanza, in her bed. He then drove to Sandy Hook Elementary School—where he attended first through fourth grades—and he killed 20 children and six adults before taking his own life, authorities said.
The 20-year-old's seclusion is proving to be the primary obstacle for investigators trying to piece together a comprehensive portrait of Mr. Lanza and his interactions with others in the days leading up to the second-worst school shooting in U.S. history, a state police official said. Afflicted with Asperger's syndrome, a form of autism, Mr. Lanza left few traces of interactions with people in person, or online, the official and others said. The person who knew him best, Nancy Lanza, is dead.
A Wall Street Journal review of public records and interviews with more than three dozen government officials, relatives and friends of the Lanza family reveal that Mr. Lanza's alienation began at a young age, causing interruptions in his education and making it more difficult for the withdrawn boy to make lasting friends.
The troubles began shortly after Peter and Nancy Lanza moved in 1998 with their young sons to Newtown, Conn., from the New Hampshire and northern Massachusetts countryside where they both grew up—a change Nancy called the family's "great adventure," said Peter's sister-in-law, Marsha Lanza. The move would be a turning point.
At the age of 6, Adam Lanza had problems soon after he started first grade at Sandy Hook Elementary School. Wendy Wipprecht, a 62-year-old freelance editor whose son, Miles Aldrich, was a classmate of Mr. Lanza, said she remembers a "couple of long talks" with Nancy Lanza.
"She was concerned about Adam," said Ms. Wipprecht, whose son is autistic. "He was clearly a very bright boy, but he wasn't doing all that well in school or somehow not comfortable in school."
By the fourth or fifth grade, Ms. Lanza pulled her son out of school to home-school him or possibly enroll him in another school, family members and friends said. "She said [the school district isn't] meeting her needs," Marsha Lanza said.
Newtown school district officials declined to release information about Mr. Lanza.
Around the time Mr. Lanza left the school, his parents separated in 2001, the person familiar with the family said. It isn't publicly known why the couple split. Peter Lanza spent time with his sons regularly after moving to Stamford, about 45 minutes from Newtown, and visited Adam every weekend until the summer of 2010, when Adam cut off ties. Peter Lanza "did a lot with his kids," Marsha Lanza said.
When Peter Lanza spoke of his son, "there was no inkling of any violence" involving Adam Lanza, said John Phillips, a friend of Peter's. He said the accused shooter's father is mystified about what could have motivated his son. "[Peter] is as clueless as everybody."
Adam Lanza was back in the Newtown school district by middle school, a silent boy usually wearing a hoodie, disconnected from his classmates, those who remember him said. But he showed no signs of violence and had no juvenile criminal record. "He wasn't a bully," said Louis Belanger, a classmate who was paired with Mr. Lanza in the 7th grade on a few class projects.
As a freshman at Newtown High School, Mr. Lanza's socially maladroit behavior attracted attention from school officials but he managed to make friends in the Tech Club and through videogames. Mr. Lanza didn't keep those connections. Later, Gloria Milas asked her son, Josh, why he hadn't returned a videogame console that belonged to Mr. Lanza. "No one knows where he is," Ms. Milas said her son told her.
Mr. Lanza's mother had again taken him out of public schools. He enrolled in classes at Western Connecticut State University. She "wanted him to have college classes," Marsha Lanza said. In Nancy's view, "he was brilliant."
But as one of the youngest people there, he was an outsider. "We tried to say hi to him every so often, and he just seemed nervous," said Dot Stasny, a classmate in an introductory German course in spring 2009.
Soon, it appeared to friends of Ms. Lanza, even Mr. Lanza's relationship with his mother was deteriorating, and she was clearly worried about him.
While Nancy, described as cheerful and generous, would go out a few nights a week to eat at a local bar, Adam remained cloistered in the home, friends said. When she went to New Orleans for a concert with Ryan, Adam stayed behind.
To bond with her son and teach him responsibility, a friend said, Ms. Lanza took Adam to shooting ranges and let him use her guns, which she had purchased legally. Some of those weapons would later be used in the Dec. 14 rampage.
In her final days, Ms. Lanza told friends she was preparing to move across the country, to Washington, where there was a school she believed would be the right fit for her son. She told her friend Mark Tambascio she would be selling her beloved Red Sox season tickets. "She was ready to move," he said.
—Alison Fox, Jacob Gershman, Matthew Dolan, Anjali Athavaley, Ted Mann, Tamer El-Ghobashy, Danny Gold, Anton Troianovski, Erica Phillips and James Oberman contributed to this article.Write to Tamara Audi at tammy.audi@wsj.com