Most children in Newtown returned to classes Tuesday for the first time since last week's massacre, but survivors of the shooting stayed home and their school remained a crime scene.

Four days after a disturbed 20-year-old shot his mother, then 20 first graders and six staff at Sandy Hook Elementary School, the debate over America's lax gun laws hit a new pitch with President Barack Obama coming out in support of a new bill that would ban assault weapons.

Adam Lanza's principal firearm in Friday's massacre was a military-grade Bushmaster semi-automatic rifle, exactly the kind of gun that would be banned if the Obama-backed bill gets past Congress -- something far from guaranteed.

But in Newtown, a small suburban town in Connecticut, there were more immediate worries, as yellow school buses rolled through thin drizzle at the delayed start of the school week.

It was only a baby step back toward normality in a town that had been known for low crime and a tight sense of community until the shootings.

Classes began up to two hours late and extra security was posted outside buildings.

At Hawley Elementary School, a couple accompanying their young son held hands and hugged the policeman at the entrance.

"He was very happy to get back with his friends," one mother said, declining to give her name. But the father said he could not describe his emotions on what should have been an ordinary school run.

"There are no words. Just tears," he said.

Another school, Head O'Meadow Elementary, was closed after an undisclosed threat, according to ABC News, which said police had turned away parents and students arriving for what would have been their first day back.

Police did not specify the threat, saying the school -- a few miles away from Sandy Hook -- was closed as a "precautionary measure," according to ABC.

Survivors of the Sandy Hook massacre stayed home.

Bill Hart, of the Newtown Board of Education, tweeted that "no date has been set" for reopening the school, adding: "We are evaluating on a daily basis."

US media reported that Newtown Superintendent of Schools Janet Robinson had sent a letter to Sandy Hook parents, inviting them to visit the former Chalk Hall school in neighbouring Monroe and saying classes would start there for survivors in January.

"We need to tend to our teachers' and students' needs to feel comfortable after this trauma in this new place," the letter said, according to Fox News and CNN. Detectives and forensic scientists continued to comb the school building in a painstaking attempt to piece together what happened when Lanza opened fire.

In a sign of the heavy hearts in this picturesque New England town, the front grills of all school buses were decorated with green and white bows, the colours of Sandy Hook school.

"The bows were hand-made overnight by the company's owners and employees," said Joan Baumgart of All-Star Transportation, which runs 50 school buses in Newtown.

Burials, which started Monday with two boys laid to rest, were continuing all week.

On Tuesday, funerals were being held for a young girl and boy, and wakes were scheduled for another boy and girl, as well as one of the teachers shot dead. They were to be buried Wednesday.

Meanwhile, police remain tight-lipped about what they've found that might explain why Lanza, who had no history of violence, snapped.

Searches have concentrated on the school, but also the house where Lanza lived with his mother Nancy -- and where he shot her at the start of his killing spree.

Among the items being examined are the rifle and pistols that Lanza carried and which were owned by his mother. There have also been reports that the hard drive to his computer is getting close attention.

Bit by bit, a picture is emerging of a boy whom no one knew well and a mother who did everything to care for him, but, fatally, introduced him to her passion for target shooting at ranges.

Former schoolmate Alan Diaz told CNN that Lanza was a "very intelligent person" who had the "stereotypical nerd look" and, unlike the backpack-toting classmates, always carried a computer bag.

"We all kind of knew that, like, he had problems socially and we kind of had a feeling that he might have had something wrong with him," Diaz said.

He recalled playing violent computer games with Lanza, but said he was surprised to hear that his friend was going shooting with his mother.

"I never really imagined Adam wanting to hold a gun," he said. "I don't imagine shy, quiet people going to a shooting range."

With the country still on edge after the Newtown rampage, nerves were rattled in the western state of Utah, where an 11-year-old boy was charged with possession of a deadly weapon after taking a gun to school, saying he was worried about protecting himself.