A heavily armed gunman opened fire on school children and staff at a Connecticut elementary school yesterday, killing at least 26 people, including 20 children, in the latest in a series of shooting rampages that have tormented the US this year.

The gunman was dead inside Sandy Hook Elementary School in Newtown, Connecticut, state police Lieutenant Paul Vance told a news conference.

Vance said there were 28 deaths in Connecticut – 20 children and seven adults from the school and another person connected to the suspect at a house in Newtown. The total included the shooter, who media said was a 24-year-old man. The New York Times reported that the gunman killed himself at the scene.

One of the dead was the shooter's mother, a teacher at the school, The New York Times reported, citing a law enforcement official.

CNN reported that an official said there was an additional death – the brother of the suspected gunman in New Jersey – but this was not independently confirmed.

The suspected gunman entered the school as children were gathered in their classrooms for morning meeting. He was armed with four weapons and wore a bullet-proof vest, WABC reported.

The gunman was dead inside Sandy Hook Elementary School in Newtown, Connecticut, state police Lieutenant Paul Vance told a news conference. Vance declined to report casualty figures but CBS News said 18 children and nine adults were dead, without clarifying whether the shooter was among those killed. The Hartford Courant reported one entire classroom was unaccounted for.

If confirmed, it would be one of the worst mass shootings in US history.

The holiday season tragedy was the second shooting rampage in the US this week and was certain to revive a debate about US gun laws.

The principal and school psychologist were among the dead, CNN said. Witnesses reported hearing dozens of shots with some saying as many as 100 were fired.

The suspected shooter, 24, was armed with four weapons and wearing a bullet-proof vest, WABC reported.

Three people were taken to Danbury Hospital, about 11 miles west of the school, a hospital spokeswoman told NBC Connecticut.

The mayor of Danbury, Mark Boughton, told MSNBC: “They are very serious injuries.”

Another person was being held in police custody after he was detained in the woods near the school wearing camouflage pants, CBS reported.

Sandy Hook Elementary School teaches children from kindergarten through fourth grade – roughly ages five to 10.

“It was horrendous,” said parent Brenda Lebinski, who rushed to the school where her daughter is in the third grade. “Everyone was in hysterics – parents, students. There were kids coming out of the school bloodied. I don’t know if they were shot, but they were bloodied.”

Lebinski said a mother who was at the school during the shooting told her a “masked man” entered the principal’s office and may have shot the principal. Lebinski, who is friends with the mother who was at the school, said the principal was “severely injured”.

Lebinski’s daughter’s teacher “immediately locked the door to the classroom and put all the kids in the corner of the room”.

A girl interviewed by NBC Connecticut described hearing seven loud “booms” as she was in gym class. Other children began crying and teachers moved the students to a nearby office, she said.

“A police officer came in and told us to run outside and so we did,” the unidentified girl said on camera.

TV images showed police and ambulances at the scene, and parents rushing toward the school. Parents were seen reuniting with their children and taking them home.

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