Police have released the names of all 20 children, aged between six and seven, who died in the Connecticut school shooting.
The state medical examiner said all the victims appeared to have been shot "more than once".
Each of the bodies examined were shot three to 11 times, H Wayne Carver, Connecticut's chief medical examiner, said on Saturday.
The gunman used a rifle as his primary weapon, Carver said.
Earlier police said the gunman, identified as 20-year-old Adam Lanza, "forced his way" into the building.
Lanza's attack on the Sandy Hook Elementary School in Newtown left 20 children and seven adults - including himself - dead.
The twenty-eighth body in the day's bloodshed was reported to be the shooter's mother, whom he is believed to have killed before driving to the school. It was first reported Lanza's mother had been a teacher at the school, but officials have told reporters there was no record of her in the school database.
In the hours after the shooting, parents at the school questioned how Lanza had been let in to the facility, and police admitted that details remain sketchy.
Connecticut state police Lt Paul Vance told reporters on Saturday that investigators had found "very good evidence" and hoped it would answer questions about the gunman's motives.
Just one person, a woman who worked at the school, was shot and survived, and Vance said her comments would be "instrumental".
Barack Obama, the US president, has meanwhile urged "meaningful action" against gun crime in the United States, but neither he nor his aides have specified what action that may entail.
"Our hearts are broken today," Obama said. "We grieve for the families of those we lost. And we keep in our prayers the parents of those who survived."
New York Mayor Michael Bloomberg, a prominent gun control proponent, declared in response that "calling for 'meaningful action' is not enough. We need immediate action. We have heard all the rhetoric before".
Residents of Newtown, a wooded small town northeast of New York City, gathered for a vigil after the massacre, filling a local church to capacity and spilling outside its doors.
Some lit candles while others joined hands to sing Christmas songs.
"This is a kind of community, when things like that happen, they really pull together," Robert Weiss, area priest, said during the Mass.
Police had earlier confused the gunman with his brother, 24-year-old Ryan Lanza, whose identity card he had reportedly been carrying.
The surviving brother was taken into custody and was being questioned, according to US television reports.
While flags in the US flew at half-mast, statements of grief and shock were made around the world.
"Afghanistan, especially, feels the pain of such incidents and almost every day such pains come to our people and we are dealing with it," Hamid Karzai, the Afghan president, said.
Britain's Queen Elizabeth II sent a message to Obama in which she said she was "deeply shocked and saddened".
Of all US campus shootings, the toll was second only to the 32 murders in the 2007 massacre at Virginia Tech.
The latest number far exceeded the 15 killed in the 1999 Columbine High School massacre, which triggered a fierce but inconclusive debate about the United States' relaxed gun control laws.