NEWTOWN, Conn. - 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 US history shattered a small Connecticut town.

A sign honoring the victims of the Sandy Hook elementary school shootings is seen in Sandy Hook, Connecticut, Dec 15, 2012. [Photo/Agencies]

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 - including first-graders, who would be about 6 years old - before turning the gun on himself.

He was also suspected of killing another person, possibly his mother, 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.

"This recovery is going to be not a sprint but a marathon. It's going to be a long process," said US Senator Richard Blumenthal of Connecticut.

Yale-New Haven Hospital opened a crisis-intervention center in the wealthy suburb of 27,000 people about 80 miles (130 km) from New York City. By mid-morning, about 50 cars were parked outside. A sign warned media to stay away.

Police and other officials stand outside Sandy Hook Elementary School in this aerial photograph a day after a mass shooting in Newtown, Connecticut, Dec 15, 2012. [Photo/Agencies]

The Connecticut school shooting moved US resident 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 US gun laws.

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

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

Latest develepment:

Identities of Connecticut shooting victims

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

Connecticut massacre victims shot multiple times

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