Agents determined Lanza's mother visited shooting ranges several times, but it's not clear whether she took her son or whether he fired a weapon there, Colbrun said.
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.
Lanza took classes at Western Connecticut State University when he was 16, and earned a B average, said Paul Steinmetz, spokesman for the school in Danbury. He said Monday that Lanza took his last class in the summer of 2009 and didn't return.
Investigators have offered no motive, 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. Authorities said Lanza had no criminal history, and it was not clear whether he had a job.
Lanza is believed to have used a Bushmaster AR-15 rifle in the school attack, 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 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.
In some of the first regulatory proposals to rise out of the Newtown shooting, Democratic lawmakers and independent Sen. Joe Lieberman said Sunday that military-style assault weapons should be banned and that a national commission should be established to examine mass shootings.