U.S. President Barack Obama has vowed to use all his power to make sure that tragedies like Friday's killing of 20 children and six adults at an elementary school are never repeated.

Mr. Obama joined mourners Sunday night in Newtown, Connecticut where a gunman's rampage killed the 26 people. He told those gathered for a nighttime vigil for the victims that they are not alone in their grief. The president said people across the country are mourning with them.

Mr. Obama said the nation is left with hard questions after the shooting. He noted this was the fourth mass shooting incident that has occurred since the start of his presidency almost four years ago.

“We can't tolerate this anymore. These tragedies must end, and to end them we must change. We will be told that the causes of such violence are complex, and that is true. No single law, no set of laws can eliminate evil from the world and prevent every single act of violence in our society. But that can't be an excuse for inaction.”

Earlier in the evening, the president met privately with the families of those who lost loved ones at Sandy Hook Elementary School, and to thank the first responders to the tragedy.

A 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 women and included the school's principal, Dawn Hochsprung, who reportedly tried to stop the shooter, and 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. An autopsy of Lanza's body is expected soon as well as an examination of his mother who reportedly was killed by Lanza at their home.

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

Connecticut state police say that social media websites claiming to have quotes from the gunman are “inaccurate.” They say 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 Polytechnic Institute and State University in Blacksburg, Virginia. Prior to that, the deadliest 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.