The three weapons found at the scene of the shooting were legally purchased by his mother, a law enforcement official familiar with the investigation told CNN.
Neighbor Gina McDade said Nancy Lanza was a "stay-at-home mom" and not a teacher or part-time employee of Sandy Hook Elementary, as some media reports stated.
Nancy Lanza had earlier worked in finance in Boston and Connecticut, said a friend who knew her well but who didn't want her name published. Nancy Lanza had retired or was on a break from her career, but she was not a teacher, the friend said.
The friend said Nancy was devoted to her sons and had been "caring for Adam," but would not provide further details.
Nancy Lanza's relatives say they share the nation's grief and struggle "to comprehend the tremendous loss that we all share," according to a statement from James Champion, who is a police officer and brother to Nancy Lanza.
"On behalf of Nancy's mother and siblings, we reach out to the community of Newtown and express our heartfelt sorrow for the loss of innocence that has affected so many," said the family statement, which was read by Rockingham County, New Hampshire, Sheriff Michael Downing.
That county includes the town of Kingston, where Adam Lanza's father, Peter, was raised.
Peter Lanza released a statement Saturday expressing condolences to the families of victims.
"Our family is grieving along with all those who have been affected by this enormous tragedy. No words can truly express how heartbroken we are. We are in a state of disbelief and trying to find whatever answers we can," said the statement.
Four years ago, the Lanzas' marriage was ending.
Nancy Jean Lanza sued Peter John Lanza for divorce on November 24, 2008 -- three days before Thanksgiving, Connecticut court records show.
The husband was known in the family as "P.J.," Marsha Lanza said.
Nancy Lanza checked off "yes" for financial disputes but "no" for parenting disputes, records show.
They were divorced in September 2009 after an uncontested hearing, records show.
Peter Lanza is tax director and vice president of taxes for GE Energy Financial Services in the New York City area, according to his resume posted on the website LinkedIn. He has been an adjunct faculty member at Northeastern University in Boston since 1995 and also teaches a partnership tax class in the master's in taxation degree program at Fairfield University in Fairfield, Connecticut, his LinkedIn page states.
On LinkedIn, he wrote summaries about himself, including: "Career dedicated to developing and refining partnership tax planning and transactional skills" and "Work closely with many of the preeminent partnership tax advisors in the United States on a daily basis."
Hours after the shooting Friday, a reporter with the Stamford Advocate found Peter Lanza as he pulled his blue Mini Cooper into his driveway in Stamford, Connecticut.
Peter Lanza was apparently unaware that his son was behind the school massacre and his ex-wife had been killed, the newspaper reported.
Peter Lanza told the reporter, "Is there something I can do for you?" and then declined to comment upon being told of his family's involvement in the shooting, the newspaper reported.
The newspaper quoted an unidentified neighbor as saying Peter Lanza and his new wife, who has been living in the neighborhood for at least a decade, were married fairly recently.
Peter Lanza was taken in for questioning, but there was no indication he would face any charges, one U.S. law enforcement official told CNN.
Ryan Lanza was taken into custody for general questioning Friday from a home in Hoboken, New Jersey, according to three law enforcement officials. They did not label him a suspect.
The more complicated story of Adam Lanza was still being assembled by authorities and media in the aftermath of the massacre.
Authorities on Saturday said they were examining the sequence of events that led Adam Lanza to dress in what a law enforcement source said was "black battle fatigues and a military vest," enter Sandy Hook Elementary and begin firing.
He was named by authorities as the invader who shot to death 20 children -- ages 6 and 7 -- and six adults, then killed himself.