NEWTOWN  - The people of Newtown, joined by President Barack Obama, poured into churches Sunday to pray for the 20 children and seven adults slaughtered in one of the worst ever US shooting massacres.

The small Connecticut town led the nation in mourning 48 hours after Adam Lanza burst into Sandy Hook Elementary School and murdered two roomfuls of six- and seven-year-old children, the school principal and five other female staff.

From early Sunday churches filled and the town Christmas tree became an impromptu place of remembrance, with people pausing every few minutes to pray and cross themselves under a light snowfall.

One middle-aged woman knelt down in front of the ranks of votive candles, teddy bears and handwritten notes, and bowed her head in tears.

“The community is gathering together and praying,” Rosty Slabicky, a Red Cross volunteer told AFP at the Catholic Saint Rose of Lima Church, where worshippers flocked to an early Mass.

“They are destroyed,” Slabicky said. “Not just the families, but the first responders are dealing with the crisis in a very personal and emotional level.”

Later Sunday, Obama was due to arrive in the leafy town to address an interfaith vigil. The White House said the president would also meet with families of victims and first responders who were sent to the carnage.

Meanwhile, the investigation entered an important new stage with the autopsy of Lanza, who is believed to have shot himself inside the school.

Coroners, who on Saturday formally identified all the school victims, were turning their attention to Lanza and also his mother, whom he murdered in her Newtown home immediately before heading to the school.

That autopsy was likely to start lifting the lid on the mystery of Lanza, who at 20 years old was seen as a withdrawn and awkward youngster, but had shown no signs of violence, let alone any indications that he might perpetrate a massacre.

Initial reports suggested Lanza used two handguns in his spree, but officials revealed Saturday that his main weapon was .223 caliber Bushmaster, a civilian version of the US military’s M4 - essentially a killing machine. Like the pistols, the rifle was registered with his mother.

Connecticut Governor Dannel Malloy revealed Sunday that Lanza blasted his way into the school, which had just installed a new security door where visitors could be viewed by video camera and buzzed in.

In addition to the mystery over Lanza, police are probing why his mother kept an arsenal of powerful weapons at her home in a well-to-do neighborhood, and how her son got his hands on them. The tragedy revived calls for stricter laws on gun ownership, particularly regarding military-style rifles, which fire bullets designed to tear a target apart, but are marketed as regular hunting weapons.

Democratic Senator Dianne Feinstein, a longtime gun-control advocate, told NBC television she would introduce a bill on the first day of the new Congress in January seeking to ban assault weapons across the country. “I’m going to introduce in the Senate, and the same bill will be introduced in the House, a bill to ban assault weapons,” Senator Dianne Feinstein of California told NBC’s “Meet the Press” program.

Many states, including Connecticut, already have strict laws on the purchase of firearms, but with no federal statutes, there is little to stop the traffic of guns from other states where fewer restrictions apply. Calls for a federal ban will not get far without strong support from Obama, who so far has made only oblique references to any action post-Newtown. “I believe he will,” Feinstein said, when asked if Obama would throw his support behind her measure.

The mayor of Connecticut’s capital city, whose own father was killed with a gun, also urged Washington on Sunday to take the lead in curbing “an incredible appetite” among Americans for guns. Speaking two days after the massacre of 20 first-grade pupils and six adults in Newtown, Pedro Segarra said Connecticut citizens are “very supportive of demilitarising our community and getting these weapons off the streets.”

But the mayor added: “A lot of this really requires action by our central government in Washington. And as long as we don’t have those efforts, some degree of decisive intervention, it’s very difficult for states to do it on their own.”

Newtown was particularly shocking because of the children killed, but this year has already seen shooting sprees at an Oregon shopping mall, a Sikh temple in Wisconsin, a movie theater in Colorado and many thousands of individual shootings across big cities.

Each year, more than 31,000 Americans die from gunshots, most of them self-inflicted, but more than 11,000 in homicides - five times as many as the death toll for US troops during an entire decade of conflict in Afghanistan.

However, with gun ownership protected by the constitution and firearms popular among a broad base of Americans, most politicians see gun bans as a vote-losing proposition.