NEWTOWN, Connecticut (Reuters) - Investigators assembled "some very good evidence" to explain what drove a 20-year-old gunman to slaughter 20 children and six adults at an elementary school, police said on Saturday, a day after one of the worst mass shootings in U.S. history shattered a small Connecticut town.

The attacker, identified by law enforcement sources as Adam Lanza, opened fire on Friday morning at Sandy Hook Elementary School in Newtown, which teaches children aged 5 to 10. The shooter killed 26 people before turning the gun on himself, and was suspected of killing another person before the massacre.

"Our investigators at the crime scene ... did produce some very good evidence in this investigation that our investigators will be able to use in, hopefully, painting the complete picture as to how - and more importantly why - this occurred," Connecticut State Police Lieutenant Paul Vance told a news conference.

The shooting has tormented the town of Newtown, once listed as the fifth-safest city in the America but now in crisis.

Yale-New Haven Hospital opened a crisis-intervention center in the wealthy suburb of 27,000 people about 80 miles from New York City. By mid-morning about 50 cars were parked outside. A sign warned media to stay away.

"If healing can come some day, that's what I want to ask God to do," said Andrea Jaeger, who placed a small candle at the makeshift memorial for the victims at the entrance to the school's driveway on Saturday morning. "Children's lives were lost and that's something we all feel."

The tragedy moved President Barack Obama to tears on national television on Friday and rattled a country that has grown accustomed to mass shootings, but not with victims so young. It also stood to revive a debate about U.S. gun laws.

"The kids who died were in two first-grade classrooms (6-year-olds)," said Mary Ann Jacob, a library clerk who described leading children out of danger during the shooting.

Vance, the police spokesman, declined to describe the evidence gathered about the shooter's motives but did say the gunman forced his way into the school, as opposed to being let in.

Crime-scene investigators worked through the night and moved the bodies to the state medical examiner's office for autopsies, police said in a statement. Victims' names had yet to be released.

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

POLICE INVESTIGATE THE GUNS

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.

"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."

Vance pointedly urged the news media to leave alone the families of the shooting victims and said a police officer had been assigned to each family "to help to maintain that solitude."

Former classmates of the shooter remembered him as a quiet loner - someone who dressed more formally than other students, often wearing khaki pants, button-down shirts and at times, a pocket protector.

"(His mother) pushed him really hard to be smarter and work harder in school," said Tim Arnone, 20, who first met Lanza at Sandy Hook.

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.

"We came here because it was going to be a good, safe community for our kids, said Catherine Hunyadi, as she and her husband wiped tears from their eyes. "You don't send your kids to school thinking something like this might happen."

Obama on Saturday urged Americans to join in solidarity as they mourned the victims, saying the hearts of parents across the country were "heavy with hurt."

Obama, in his weekly radio and Internet address, called for "meaningful action to prevent more tragedies like this," but stopped short of specifically calling for tighter gun-control laws.

The president wiped away tears in a television address on Friday, telling the nation, "Our hearts are broken."

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.

It 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 U.S. school shootings, the 1999 rampage at Columbine High School in Littleton, Colorado, where two teenagers murdered 13 students and staff before killing themselves.

(Additional reporting by Edith Honan, Dave Gregorio, Chris Francescani, Ernest Scheyder and Edward Krudy; Writing by Daniel Trotta; Editing by Will Dunham)