The woman found dead at the secondary crime scene was related to the shooter, a police news release said. Many media outlets have reported she was the shooter's mother, Nancy Lanza.

Nancy Lanza legally owned a Sig Sauer and a Glock, both handguns of models commonly used by police, and a military-style Bushmaster .223 M4 carbine, according to law enforcement officials who also believe Adam Lanza used at least some of those weapons.

Undated photo provided by a family member to ABC News reportedly shows Nancy Lanza. Lanza was the mother of suspected gunman Adam Lanza involved in the Sandy Hook Elementary School shooting in Connecticut. [Photo/Agencies]

"We're investigating the history of each and every weapon, and we will know every single thing about those weapons," Vance said.

Nancy Lanza was an avid gun collector who once showed him a "really nice, high-end rifle" that she had purchased, said Dan Holmes, owner of a landscaping business who recently decorated her yard with Christmas garlands and lights. "She said she would often go target shooting with her kids."

Newtown was ranked the fifth safest city in America by the website NeighborhoodScout.com based on 2011 crime statistics.

"This wonderful town that we all love for its peace, beauty, the great schools - all of that - has become Columbine," said Julie Maxwell Shull, a sixth-grade teacher at Reed Intermediate School, referring to the high school that was site of a 1999 shooting in Colorado.

Ryan Lanza, brother of suspected school shooter Adam Lanza, is driven from One Police Plaza in Hoboken, New Jersey, Dec 14, 2012. [Photo/Agencies]

The holiday season tragedy was the second shooting rampage in the United States this week and the latest in a series of mass killings this year.

In another incident, Oklahoma police arrested an 18-year-old high school student on charges that he was plotting to carry out a shooting and bombing massacre at his school, according to the Tulsa World newspaper.

Police said in an affidavit Chavez tried to recruit other Bartlesville High School students to help him lure classmates into the school auditorium, where he planned to chain the doors shut and start shooting them, the newspaper said.

The Connecticut massacre revived a debate about gun-control in a country with a flourishing firearms culture and a strong lobby that has discouraged most politicians from any major efforts to address the easy availability of guns and ammunition.

The death toll exceeded that of one of the most notorious US school shootings, the 1999 rampage at Columbine High School in Littleton, Colorado, where two teenagers murdered 13 students and staff before killing themselves.

Latest develepment:

Identities of Connecticut shooting victims

12 girls, 8 boys, 6 women killed in Connecticut school shooting

Connecticut massacre victims shot multiple times

US elementary school shooting kills 20 kids

Reactions on US gun control:

Obama urges solidarity as America mourns shooting victims

Obama: US needs to take actions to prevent shooting

School massacre pressures Obama on US gun control

Condolences:

President Hu sends condolences to Obama over school shooting

UN chief mourns victims of US school shooting

EU leaders shocked by US school shooting

Background:

Major school shootings in US since 1999