Shattered families and grieving residents struggled Saturday to grasp the news that most of the 27 people shot dead by a US school gunman were six- and seven-year-old children.

President Barack Obama was due to join the vigils in the small Connecticut community of Newtown on Sunday, to lead national mourning after this latest massacre that has revived calls for a debate on gun control.

But the political ramifications of the tragedy were far from the minds of most in this picturesque dormitory town, where parents of the survivors and the dead alike were struggling to come to terms with the stunning loss.

Robbie Parker, a 30-year-old hospital physician's assistant who cares for sick newborns, said the death of his loving six-year-old Emilie should "inspire us to be better, more compassionate and caring toward other people."

And he included the family of the apparent shooter, 20-year-old Adam Lanza, in his condolences, addressing them through the news media to say: "I can't imagine how hard this experience must be for you."

Robert and Diane Licata described how their six-year-old son Aiden ran past the shooter in his classroom doorway to escape after seeing his teacher gunned down -- and recounted their desperate search for him.

Diane told CNN that she had rushed to the school and seen her daughter led out of the building, but there was no sign of Aiden.

"So the kids start to come out and when I saw her, you know, the sense of relief is incredible, but it's really short-lived because I still have one in there. And I'm waiting for him to come. And he didn't come out," she said.

"And I didn't see his teacher and I didn't see his classmates," Diane added. "I hugged my daughter and I said, you know -- I told her to hug her friend who was in her class and I said go to the fire station, Daddy is coming.

"And I'm going to wait for Aiden. I'm going to wait. And he didn't come out. So you know, when you're standing there waiting, and no-one will tell you anything. It's an indescribable feeling of helplessness."

Licata eventually received a text that her son was safe at a nearby police station, and he was later able to explain his escape.

"They heard noises that he described as initially they thought were hammers falling. Then they realized that it was gunshots," she said.

"And... Aiden's teacher, had the presence of mind to move all of the children to a distance away from the door on the side of the room furthest away from the door and that's when the gunman burst in, did not say a word.

"No facial expressions. And proceeded to shoot their teacher."

Following drills that many US children are taught as to how to react in the presence of a gunman, Aiden and his classmates rushed to the door where he was standing and ran past him. Some of them survived.

"He really, really, really cared about his teacher. He was very close with her and she really loved that class. He keeps saying, I really hope she's okay, I hope it's not her," said Diane.

"He knows that she's been hurt but he doesn't know the end result. He knows the kids that he saw getting shot. He doesn't know the outcome."

A police spokesman said Lanza is believed to have shot his mother at the home they shared before heading to the nearby school and launching his attack.

He had two handguns, but the coroner told reporters that most of the children and staff were killed by multiple gunshots from his assault rifle, a military-style .223 caliber Bushmaster, similar to the US Army's M4.

Connecticut Chief Medical Examiner H. Wayne Carver said such a rifle fires rounds that are "designed in such a fashion that the energy is deposited in the tissue, so the bullet stays in."

Connecticut State Police, meanwhile, released the ages of the victims, who included 16 six-year-olds and four seven-year-olds.

Twelve of the 20 slain children were girls and eight were boys.

The six adults killed were all women, including the school principal and the school psychologist, who at 56 was the eldest victim.

The motives of the shooter were the biggest mystery. Carver said he would conduct a post-mortem on Lanza and on his mother later Saturday.

Connecticut State Police spokesman Lieutenant J. Paul Vance said detectives had begun to "peel back the onion."

Asked whether any suicide note, emails or other clues to the killer's mind had been found, he said the crime scene "did produce some very -- very good evidence in our investigation."

Vance said the crime scene investigation could go on for 24 to 48 hours.

Bodies were removed from the school overnight and relatives were privately given formal identification of the dead.

Although he was remembered as a shy, awkward and nerdy boy, Lanza had not apparently given any warning sign that he was a mass murderer.

The weapons, news reports said, were registered in his mother's name, but she was widely seen as an unremarkable and upstanding resident in the town.

The tragedy drew messages of support from around the world, and candlelight vigils have sprung up in the area.

Of all US campus shootings, the toll was second only to the 32 murders in the 2007 massacre at Virginia Tech university.

The latest number far exceeded the 15 killed in the 1999 Columbine High School massacre, which triggered a fierce but inconclusive debate about relaxed gun control laws in the United States.

However, the White House has scotched any suggestion that the politically explosive subject would be quickly reopened.