Adam Lanza, the 20-year-old who slayed 26 people, including 20 children, and his mother both spent time at a gun range in Connecticut, said US media reports.
Investigators have determined Lanza did visit a gun range, but they have not determined whether he shot there, officials probing the second-deadliest school shooting in US history said.
Investigators have also learned his mother, Nancy Lanza, visited a gun range on multiple occasions, but they have not determined whether her son was with her during those visits, a spokesperson of the Bureau of Alcohol, Tobacco, Firearms and Explosives was quoted by the ABC News as saying.
ATF agents have been canvassing area gun ranges and gun dealers to learn whether Adam Lanza had been a customer or a visitor.
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 2007, a gunman had killed 32 people in a massacre in Virginia Tech, the deadliest shooting incident in the country.
Those who knew the shooter struggled to reconcile the difference between the quiet, withdrawn youngster without a criminal record and the man who donned black fatigues and a military vest and rained hell at Sandy Hook Elementary School last week.
Police identified Adam Lanza as the shooter and that he killed his 52-year-old mother, Nancy, in their home before walking into the school and spraying bullets, killing 26 more people -- 20 of them children no older than seven.
The rampage ended when Lanza apparently took his own life in a classroom. He was carrying 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 are yet to disclose a motive for the attack -- which left those who knew Lanza trying to discern whether anything in his past could have foreshadowed the present, CNN reported.
Lanza moved to Connecticut from Kingston, New Hampshire, with his parents and older brother Ryan, according to a booklet for the town's Newtown's Bennetts Farm neighbourhood.
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 after that for him isn't clear, except that he continued to lived in the picturesque, 300-year-old Connecticut town.
His father, Peter, remarried and lived not far from Newtown, an official said. He was questioned by authorities after the shooting. So, too, was Adam's 24-year-old brother, Ryan.
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," he said.
"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," he added.
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 ended up having her son home-schooled.
"She had issues with school," said the aunt, who lives in Crystal Lake, Illinois. "I'm not 100 per cent certain if it was behaviour or learning disabilities, but he was a very, very bright boy. He was smart."
"You could definitely tell he was a genius," Alex Israel, a classmate of Lanza at Newtown High School who lived a few houses down from him, was quoted by the channel as saying.
"He was really quiet, he kept to himself," Israel added.
A relative told investigators that Adam 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.