Federal agents have concluded that Lanza visited an area shooting range, but they do not know whether he actually practiced shooting there.
Ginger Colbrun, a spokeswoman for the federal Bureau of Alcohol, Tobacco and Firearms, would not identify the range or say how recently he was there.
Agents also determined that Lanza's mother visited shooting ranges several times, but it's still not clear whether she brought her son to the range or whether he ever fired a weapon there, Colbrun said.
Investigators have offered no motive for the shooting, and police have found no letters or diaries that could shed light on it. They believe Lanza attended Sandy Hook many years ago, but they couldn't explain why he went there Friday.
A law enforcement official, speaking on condition of anonymity, said investigators are reviewing the contents of Lanza's computer, as well as phone and credit card records in an effort to piece together his activities leading up to the shooting. The official was not authorized to discuss the details of the case.
Newtown police Lt. George Sinko said he "would find it very difficult" for students to return to the same school where they came so close to death.
But, he added, "We want to keep these kids together. They need to support each other."
Jennifer Waters, who at 6 is the same age as many of the dead but attends another school, came to Mass at Saint Rose of Lima Roman Catholic church with lots of questions.
"The little children — are they with the angels?" she asked her mother.
Joan Waters assured her daughter that they were, then hushed the child as services continued with boxes of tissues placed in each pew and window sill.
An overflow crowd of more than 800 people packed the church where eight children will be buried this week. Lanza and his mother also attended the church. Spokesman Brian Wallace said the diocese has yet to be asked to provide funerals for either.
In his homily, the Rev. Jerald Doyle tried to answer the question of how parishioners could find joy in a holiday season with so much sorrow.
"You won't remember what I say, and it will become unimportant," he said. "But you will really hear deep down that word that will finally and ultimately bring peace and joy. That is the word by which we live. That is the word by which we hope. That is the word by which we love."
At a later Mass at St. Rose of Lima, the priest stopped midway through the service and told worshippers to leave, because someone had phoned in a threat. Police searched the church and the rectory but found nothing dangerous.
The rifle used in Friday's attack was a Bushmaster .223-caliber, a civilian version of the military's M-16 and a model commonly seen at marksmanship competitions. It's similar to the weapon used in the 2002 sniper killings in the Washington, D.C., area and in a recent shopping mall shooting in Oregon.
Versions of the AR-15 were outlawed in the United States under the 1994 assault weapons ban. That law expired in 2004, and Congress, in a nod to the political clout of the gun-rights lobby, did not renew it.
Authorities said Lanza had no criminal history, and it was not clear whether he had a job.
At least one law enforcement official has said Lanza had been diagnosed with Asperger's, a mild form of autism often characterized by social awkwardness.
People with the disorder are often highly intelligent. While they can become frustrated more easily, there is no evidence of a link between Asperger's and violent behavior, experts say.
Associated Press writers John Christoffersen and Michael Melia in Newtown, David Collins in Hartford and Brian Skoloff in Phoenix contributed to this report.