TWENTY children have died after a gunman opened fire at an elementary school in one of America's worst mass shootings.

The gunman's mother, a kindergarten teacher at the school, was among the victims.

Officials said the killer was 20-year-old Adam Lanza of New Jersey who died at the scene of the horrifying early-morning shooting spree at Sandy Hook Elementary School in Newtown, Connecticut.

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Witnesses said he went from class to class shooting at pupils and staff, killing his mother Nancy, who sources said appeared to be the primary target.

He is said to have fired at least 100 rounds with several weapons.

Police said at least 28 people, including eight adults, were killed and many of the children were in class when they were shot.

The gunman's father was found dead at home in Newtown, officials said.

Lanza's 24-year-old brother Ryan, of Hoboken, New Jersey, was being questioned by police.

Another body was found at a separate crime scene in Connecticut, with unconfirmed reports indicating he was another of the gunman's brothers.