US police have released the names of 26 people shot dead at a Connecticut elementary school yesterday, including 20 children aged six and seven, in an incident that stands as one of the worst mass shootings in US history.

Six adults, aged 27 to 56 and all female, were identified by authorities a day after a heavily armed 20-year-old gunman forced his way into the Sandy Hook Elementary School.

Among the children, there were eight boys and 12 girls.

The gunman used a long rifle as his primary weapon, the state's chief medical examiner told reporters, adding that "all the wounds that I know of" were caused by a rifle.

Asked to describe the attack, Chief Medical Examiner Dr H Wayne Carver II, who oversaw the autopsies of all the victims and conducted many himself, called it "the worst I have seen."

"They were wearing cute kid stuff," he said when asked to describe the children he examined. "They were first graders. It's the kind of stuff you'd send your kids or your grandkids out the door to first grade in."

A total of 28 people died in the rampage, including the gunman.

The list of names released did not include the shooter, identified by law enforcement sources as Adam Lanza, or his mother, who Dr Carver confirmed was killed at the nearby crime site authorities had previously disclosed.

The attack, which ended when the gunman turned his weapon on himself, stunned members of the tight-knit suburban community, once listed as the fifth-safest town in the America but now in crisis.

Police earlier said they had assembled "some very good evidence" on the killer's motives.

"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 reporters.

Townsfolk set up multiple small memorials near the school, one made of flowers assembled around the school's sign - which reads "Visitors Welcome" - and another, further down Riverside Road where people put up more than two dozen candles ranging from small household votives to large, ceremonial candles.

Yale-New Haven Hospital opened a crisis-intervention centre in the wealthy suburb of 27,000 people about 130km from New York City.

US President Barack Obama, who a day earlier was moved to tears on national television by the tragedy, called for "meaningful action to prevent more tragedies like this," but stopped short of specifically calling for tighter gun-control laws.

The nation has experienced many mass shootings, but rarely have the victims been so young. The incident also stood to revive a debate about US gun laws.

The killer's mother, 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.

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.

Horror over extent of fatalities

World leaders expressed horror as they offered condolences to the United States.

Pope Benedict conveyed his "heartfelt grief" through Vatican Secretary of State Cardinal Tarcisio Bertone.

"In the aftermath of this senseless tragedy, he asks God our Father to console all those who mourn and to sustain the entire community with the spiritual strength which triumphs over violence by the power of forgiveness, hope and reconciling love," Cardinal Bertone said.

German Chancellor Angela Merkel said, "The news from Newtown saddens me deeply. Once again, we're completely aghast over an act that we can't comprehend. An incredible suffering has been brought to so many families so close to Christmas."

British Prime Minister David Cameron said via Twitter that he was "devastated" by the shootings and said the death of so many children was "truly heartbreaking".