Connecticut State Police walk near the scene of an elementary school shooting on December 14, 2012 in Newtown, Conn. Credit: Getty

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A heavily armed gunman massacred more than two dozen people, including 18 young children, at a suburban Connecticut elementary school Friday, US media reported, citing law enforcement officials.

Connecticut State Police spokesman Lieutenant Paul Vance could not confirm the death toll, but told reporters there had been “several fatalities” among staff and students at the Sandy Hook Elementary School in Newtown, Connecticut.

He said the shooter had died within the building during a police operation to rescue the pupils, and that the scene was now secure and the public safe, but gave few details of the latest in a grim series of US mass shootings.

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US media quoted police saying the shooter was 24-year-old Ryan Lanza and that he’d shot his mother, who taught at the school, and many of her students.

According to the Hartford Courant newspaper, many of the shots were fired in a kindergarten classroom. The child victims were aged between five and ten.

If confirmed, the toll would be the second highest death toll in a US school shooting, after the 2007 campus shootings at Virginia Tech, which left 32 dead.

The number would far exceed the 15 killed in the 1999 Columbine High School massacre, which triggered a fierce but inconclusive debate about the United States’ relaxed gun control laws.

CBS News cited law enforcement sources as saying that 27 people were killed, including 18 children. Other reports put the death toll as much as 30.

Before going to the school, the killer had first shot down his father, US media reported, citing unnamed police sources.

A tearful President Barack Obama went on national television to express his “overwhelming grief.” He ordered flags to be lowered at half mast.

Witnesses described a terrifying bloodbath in the small town school, shortly after classes had got underway.

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“I was going back to my classroom and I heard like a person kicking on the door and I turned around I smelled smoke,” an eight-year-old boy told NBC.

“Then bullets whizzed by and then a teacher pulled me into her room,” he said, describing “total panic.”

Other witnesses described an intense fusillade, with perhaps 100 rounds fired, and seeing a corridor splattered with blood.

“I was in the gym at the time … we heard lots of bangs, and we thought that it was the custodian knocking stuff down. We heard screaming. And so went to the wall, and we sat down,” a young boy told WCBS television.

“Then the police came in. It’s like, is he in here? Then he ran out. Then somebody yelled get to a safe place, so we went to the closet in the gym and we sat there for a little while,” he said, as stunned parents arrived.

“Then the police like were knocking on the door, and they’re like, we’re evacuating people, we’re evacuating people. We ran out.

“They’re police at every door leading us down this way, this way. Quick, quick, come on. We ran down to the firehouse. There’s a man that pinned down to the ground with handcuffs on,” he said.

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Police swarmed into the leafy neighborhood after the shooting, while other area schools were put under lock-down, police and local media said.

A photo on the Newtown Bee newspaper’s website showed officers leading more than a dozen frightened small children across a parking lot.

Deadly shootings are a frequent occurrence in US public places, often ending only when the gunman is shot or kills himself.

On Tuesday, a man with a semi-automatic rifle raked an Oregon shopping mall, killing two people, then taking his own life.

In the most notorious recent incident until now a 24-year-old, James Holmes, allegedly killed 12 people and wounded 58 others when he opened fire in a midnight screening of the latest Batman movie in Aurora, Colorado, in July.

Last month, gunman Jared Loughner was jailed for life for killing six people in Tucson, Arizona, in January 2011 in an attack targeting congresswoman Gabrielle Giffords, who was shot in the head at point-blank range but survived.

However, despite the tragedies, support for tougher gun ownership laws is mixed, with many Americans opposing restrictions on what they consider to be a constitutional right to keep powerful firearms at home.