A day after a lone attacker went on a shooting spree at a Connecticut school killing 20 kids and six adults, US police and law enforcement agents were today trying to establish the motive behind his actions and questioned his elder brother and father.

Initially, local media reports and law enforcement agents had identified the gunman as 24-year old Ryan Lanza, who eventually turned out to be the elder brother of the attacker Adam Lanza.

20-year-old Adam, after killing his mother at home, opened indiscriminate fire in the Sandy Hook Elementary School in Newtown and gunned down 20 children aged between 5 and 10 as well as six adults, including the institute's Principal.

Adam, who was wearing black combat gear and armed with semi-automatic pistol and a semi-automatic rifle, later died from a self-inflicted gun shot wound, law enforcement agents said. He was found dead inside the school building.

As news of the shooting broke, Ryan wrote on his Facebook page that he was being wrongly identified as the gunman.

"I'm on the bus home now it wasn't me... It wasn't me I was at work it wasn't me (sic)," the elder Lanza said.

Adam's mother Nancy was a teacher at the school where the shooting occurred, media reports said.

The New York Times said he had first killed her in her apartment before gunning down 26 people at the school.

Connecticut Police spokesman Paul Vance told reporters outside the scene of shooting that Adam drove to the school where he opened fire in two classrooms located in one section of the building. He was carrying three guns, including a Sig Sauer pistol and a .223-caliber rifle, which was found in the back of a car.

FBI agents interviewed his brother Ryan in New Jersey's Hoboken area as well his father Peter Lanza, who was divorced from his mother.

Yesterday's incident came less than two weeks before Christmas and is being considered the US' second-deadliest school shooting. Earlier in 2007, a gunman had killed 32 people in a massacre in Virginia Tech.

In providing details of the shooting, Vance said law enforcement agents found "a very horrific and difficult scene" at the school, which had 700 students in kindergarten.

Vance said 18 children died in the school building while two were declared dead at a local hospital after they were rushed their for medical treatment.

In his last briefing to reporters late last night, Vance said his officers were working on "preparing a list of positive identifications." He did not reveal the names of the victims.

The school's principal Dawn Hochsprung and a psychologist were among those dead and teachers resorted to various ways to protect the children, asking them to take cover under their desks and locking some children in the school's gym and classroom closets.

School librarian Yvonne Cech said she had spent 45 minutes locked in a closet with two library clerks, a library catalog assistant and 18 fourth-graders.

"The SWAT team escorted us out," she was quoted as saying by the New York Times.

Panicked and weeping parents were seen rushing to the school and were gathered at a staging area near the school as the children were led out in a single file from the school.

A woman at the scene told CNN that "20 parents were told that their children are dead. It was awful."

The New York Times said Adam was remembered by his high school classmates as smart, introverted and nervous. They said he had gone out of his way not to attract attention when he was younger.

Candle-light vigils were held in Newtown and across cities in America as the nation stood in solidarity with the families of the young children.

Obama also ordered that the national flag will be flown half mast in honour of the victims.

Adam's neighbour, Beth Israel, 43, said that she and her family lived down the street from the family but had not spoken to them in three years.

"He was a socially awkward kid," she said of Adam. "He always had issues. He was kind of a loner. I don't know who his friends were. He kind of kept to himself."