NEWTOWN, Conn. - Twenty schoolchildren were slaughtered by a heavily armed gunman who opened fire at a suburban elementary school in Connecticut on Friday in one of the worst mass shootings in US history.

The gunman - who according to a media report carried four weapons and wore a bulletproof vest - was dead inside Sandy Hook Elementary School in Newtown, Connecticut, state police Lieutenant Paul Vance told a news conference.

A young boy is comforted outside Sandy Hook Elementary School after a shooting in Newtown, Connecticut, December 14, 2012. [Photo/Agencies]

Vance said authorities found 18 children and seven adults, including the gunman, dead at the school, and two children were pronounced dead later after being take to a hospital. Another adult was found dead at a related crime scene in Newtown, he said, bringing the toll to 28.

"Our hearts are broken today," President Barack Obama said in an emotional televised address to the nation.

"Evil visited this community today," Connecticut Governor Dannel Malloy told reporters.

Two law enforcement sources briefed on the investigation confirmed to Reuters the shooter had been identified as Adam Lanza, 20. Adam's brother Ryan Lanza was "either in custody or being questioned" at this hour, one of the sources said.

The New York Times reported that the gunman walked into a classroom where his mother was a teacher, shot his mother and then 20 students, most in the same classroom, b e fore shooting five other adults and killing himself. One other person was shot at the school and survived, the Times said.

The holiday season tragedy was the second shooting rampage in the United States this week and the latest in a series of mass killings this year, and was certain to revive a debate about US gun laws.

Chaos struck as children gathered in their classrooms for morning meetings at Sandy Hook Elementary in Newtown, a city of 27,000 in Fairfield County, about 80 miles (130 km) northeast of New York City.

Police swarmed the scene and locked down the school, rushing children to safety, some of them bloodied. Distraught parents converged, frantically searching for their daughters and sons. Neighbors and friends wandered in shock, looking for information.

"It's hard to believe that anything like this could happen in this town," said resident Peter Alpi, 70, as he fought back tears. "It's a very quiet town. Maybe it's too quiet."

Hours later, Obama, wiping away tears and pausing to collect his emotions, mourned the "beautiful little kids between the ages of 5 and 10 years old" who were killed. He ordered flags flown at half staff at US public buildings.

"As a country, we have been through this too many times," Obama said, ticking off a list of recent shootings.

"We're going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics," Obama said in apparent reference to the influence of the National Rifle Association over members of Congress.

Obama remains committed to trying to renew a ban on assault weapons, White House spokesman Jay Carney said.

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