Authorities said it's also not clear whether Lanza entered before or after 9:30 a.m., the time each day when the school would lock its doors as part of a security system introduced this year. Authorities say the first emergency call about the shooting came in at "approximately" 9:30 a.m. Friday.
Within minutes, 26 people had been killed with chilling efficiency, leaving only the one wounded survivor. The survivor, an adult, has not been named.
"She has been treated and she'll be instrumental in this investigation, as I'm sure you can understand," Vance said.
The massacre in Newtown is the second-deadliest school shooting in U.S. history, behind the 2007 Virginia Tech mass shooting that left 32 dead.
"Stuff like this does not happen in Newtown," said Renee Burn, a teacher at another school in the town, which is roughly 75 miles northeast of New York City.
Until Friday, only one homicide in the past 10 years had been reported in the upscale community of expansive homes surrounded by woods, where many residents commute to jobs in Manhattan and the nearby Connecticut cities of Stamford and Hartford.
Flags were lowered to half-staff in a number of states, and vigils were held at houses of worship and at schools amid a national outpouring of grief.