Some details were issued on Saturday about the killings at Sandy Hook school in Newtown, Connecticut, where 20 children and six adults were shot.
The gunman, named as Adam Lanza, 20, killed his mother before driving to the school and opening fire shortly after 9:30am Friday (local time).
The BBC reports the killings took place in two rooms. The shootings lasted a few minutes before he shot himself.
All the bodies have been removed from the school.
A list of the dead has been issued. All the children were aged six or seven and all the adults were women. Most of the children were girls.
Chief medical examiner H Wayne Carver II said they all died of gunshot wounds.
Dr Carver said all were shot more than once, with a rifle.
Lieutenant Paul Vance of Connecticut state police said the gunman forced his way into the school and was not let in "voluntarily".
He said investigators had to "peel back the onion, layer by layer and examine every crack and crevice" of the school.
"The detectives will analyse everything and put a complete picture together of the evidence that they did obtain. We're hopeful, we're hopeful that'll paint a complete picture as to how and why this entire incident, unfortunate incident occurred," he told a news conference.
The principal of the school was killed as she lunged at the gunman, trying to overpower him.
He drove to the school in his mother's car and was wearing a bullet-proof vest. The guns were reportedly registered to her.
She legally owned a Sig Sauer handgun, a Glock handgun and a Bushmaster .223 M4 carbine.
There are conflicting reports about whether she had worked at the school in the past.
Lieutenant Vance said vital clues had been found at the school and the home of the gunman's mother.
Nation in mourning
Flags in the United States will be flown at half-mast until Tuesday in honour of those killed.
Prayer vigils have been held in the village of Sandy Hook and across the country.
President Barack Obama is to visit Newtown to meet families and speak at a vigil.
After the attack, he urged "meaningful action" against gun crime in the US.
"As a country we have been through this too many times," he said in an emotional White House address.
Grieving residents have held candlelight vigils and attended church services to remember the victims of the one of the worst mass shootings in American history.
The ABC reports signs hang outside homes and businesses in Newtown. Some read "God Bless Sandy Hook", another reads "Hug a teacher today".
Police officers remain stationed outside the homes of each of the families devastated by the tragedy.
Connecticut Governor Dannel Malloy made a television address saying the children were "taken from their families far too soon".
"What's important right now is love, courage and compassion," he said.
The death toll exceeds that of the 1999 killings at Columbine High School in Littleton, Colorado, where two teenagers murdered 13 students and staff before killing themselves.