U.S. authorities have yet to say what might have been the motive of a 20-year-old gunman in a mass shooting that killed 20 young children and six adults at an elementary school in the town of Newton, Connecticut.

Hundreds of mourners descended on a church in Newtown Friday night to grieve the deaths, as the community and the nation sought to absorb the scope of the tragedy. Governor Dannel Malloy told the mourners “evil visited this community today.”

Earlier, in Washington, a tearful U.S. President Barack Obama addressed the American public, saying “our hearts are broken” by the deaths.

His address came hours after horror and mayhem struck at the school in the small community 130 kilometers northeast of New York. The president said, “The majority of those who died today were children — beautiful little kids between between the ages of five and 10 years old.”

Witnesses say the gunman dressed in black military fatigues entered the school at mid-morning Friday, opening fire on children and their teachers in two rooms. The names of the young victims have yet to be released. The principal of the school was among those killed.

A military assault rifle was later found in the gunman's car. The shooter, identified as Adam Lanza, died at the scene of a self-inflicted gunshot. Two semi-automatic handguns were found nearby.

Authorities say the shooter killed his mother at her home where he also lived, then drove her car to the school where witnesses say he opened fire without saying a word.

The gunman's brother was taken by police for questioning late Friday in New Jersey. Police say he is cooperating in the probe.

President Obama called the shooting a “heinous crime,” and said “as a country we have been through this too many times.” He then listed a string of recent mass shootings, including those at a temple in Wisconsin, a movie theater in Colorado and a shopping mall in Oregon. He said “we're going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics.”

Video from the crime scene showed children being rushed from the school single file, people hugging, and frantic parents either waiting for word on their children or hugging them once found. It took several hours before the full scope of the tragedy became public.

When it did, reports of heroism surfaced as well. In one instance, a teacher herded her entire first grade class into a bathroom, locked the door, and cowered alongside the children until help arrived.

When police arrived on the scene, she made them slide identification under the door before she would open it.