The United States was in mourning Sunday for an attack that killed 27 people including children in Newtown.

The United States was in mourning Sunday for an attack that killed 27 people including children in Newtown.

The people of Newtown, soon to be 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.

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That autopsy was likely to start lifting the lid on the mystery of Lanza, who at 20 years old was seen as a withdrawn and awkward youngster, but had shown no signs of violence, let alone any indications that he might perpetrate a massacre.