U.S. President Barack Obama has arrived in Newtown, Connecticut to join in the mourning of 20 young children and six adults shot dead in a gunman's rampage on Friday.

Mr. Obama is expected to meet with the families of those who lost loved ones at Sandy Hook Elementary School, and thank first responders to the tragedy. The president also plans to speak at a nighttime vigil in the small affluent town about 120 kilometers northeast of New York City.

An official list of the victims includes the names of 12 girls and eight boys – all of them in the same grade and ages six to seven years old. The adult victims were all women and included the school's principal, Dawn Hochsprung, who reportedly tried to stop the shooter, and 27-year-old teacher Victoria Soto, who is credited with saving some of her students by putting herself between them and the attacker.

At midday Sunday, Connecticut state police there are still “weeks worth of work” left in the investigation, and that they will not release a motive for the killing before they “have the whole picture.”

Authorities have identified the killer as Adam Lanza, who is described as a quiet and socially awkward, but very bright 20-year-old. Officials say Lanza died at the scene of a self-inflicted gunshot wound. They expect to complete a full autopsy on him later Sunday, as well as an examination of his mother reported to have been killed by her son at their home.

Connecticut's chief medical examiner said the victims were all killed at close range by multiple gunshots from an assault rifle. According to law enforcement officials, Lanza's mother legally owned the assault rifle used by her son in the massacre, as well as two handguns reported found near his body.

Connecticut state police also announced Sunday that social media websites claiming to have quotes from the gunman are “inaccurate.” They said authorities have deemed the online information as “threatening” and will prosecute anyone posing as the shooter.

Friday's attack was the second worst school shooting in U.S. history. In 2007, a gunman killed 32 people at Virginia Tech University in Blacksburg, Virginia. Prior to that, the most notorious U.S. school shooting was the 1999 rampage at Columbine High School in Littleton, Colorado, where two teenagers killed 13 students and staff before killing themselves.