"She is doing fine," Vance said at a news conference today. "She has been treated and she'll be instrumental in this investigation."
Vance said it appears that reports of an altercation involving Lanza at the school in the days before the mass slaying are not checking out.
Vance said that Lanza forced his way into the school, but did not say how.
Evidence emerged today that Lanza's rampage began in the office of school principal Dawn Hochsprung while the school intercom was on. It's not clear whether it was turned on to alert the school or whether it was on for morning announcements, but the principal's screams and the cries of children heard throughout the school gave teachers time to take precautions to protect their children.
Hochsprung was among those killed in the Friday morning killing spree.
Authorities have fanned out to New Jersey, Connecticut and Massachusetts to interview Lanza's relatives, ABC News has learned.
According to sources, Lanza shot his mother in the face, then left his house armed with at least two semi-automatic handguns, a Glock and a Sig Sauer, and a semi-automatic rifle. He was also wearing a bulletproof vest.
Lanza then drove to the elementary school to carry out his murderous plan, authorities said.
It appeared that Lanza died from what was believed to be a self-inflicted gunshot wound.
Lanza's father, Peter Lanza, released a statement today saying the family is "grieving along with all those who have been affected by this enormous tragedy."
"No words can truly express how heartbroken we are," he said. "We are in a state of disbelief and trying to find whatever answers we can. We too are asking why. We have cooperated fully with law enforcement and will continue to do so. Like so many of you, we are saddened, but struggling to make sense of what has transpired."
This is the second worst mass shooting in U.S. history, exceeded only by the Virginia Tech shooting in 2007 when 32 were killed before the shooter turned the gun on himself. The carnage in Connecticut exceeded the 1999 Columbine High School shooting in which 13 died and 24 were injured.
Friday's shooting came three days after masked gunman Jacob Roberts opened fire in a busy Oregon mall, killing two before turning the gun on himself.
The Connecticut shooting occurred at the Sandy Hook Elementary School, which includes 450 students in grades K-4. The town is located about 12 miles east of Danbury, Conn.
ABC News' Emily Friedman, Michael S. James and Shushannah Walshe contributed to this report.