A heavily armed gunman opened fire at a Connecticut elementary school on Friday, killing himself and another 26 people, including 20 children, in the latest in a series of shooting rampages across the United States this year, U.S. media reported.

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

Vance said there were 28 deaths; 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. U.S. media first reported that the gunman was 24-year-old Ryan Lanza, from New Jersey.

However, law enforcement officials speaking on condition of anonymity told the Associated Press that the shooter was in fact his younger brother Adam Lanza, 20, and that Ryan was being questioned by police. Earlier, a law enforcement official mistakenly transposed the brothers' first names.

One of the officials said that Adam Lanza has been killed from a self-inflicted gunshot wound. The shooter's mother, Nancy Lanza, a teacher at the school, is also presumed dead, another official said. 

The 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.

If confirmed, it would be one of the worst mass shootings in U.S. history. The holiday season tragedy was the second shooting rampage in the United States this week and was certain to revive a debate about U.S. gun laws.

President Barack 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."

"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.

Witnesses reported hearing dozens of shots with some saying as many as 100 were fired.

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 5 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."

Television 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.

"This is going to be bad," a state official told Reuters, requesting anonymity because the scope of the tragedy remained uncertain.

President Barack Obama was notified and would receive regular updates throughout the day, White House spokesman Jay Carney said.

"We're still waiting for more information about the incident in Connecticut," Carney said when asked about the president's reaction to it.

Carney called the event "tragic" and said there would be time later for a discussion of policy implications. 

All Newtown schools were placed in lockdown after the shooting, the Newtown Public School District said.

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."

Danbury Hospital, about 11 miles west of the school, had received three patients from the scene, a hospital spokeswoman told reporters.

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.

Newtown, with a population about 27,000, is in northern Fairfield County, about 45 miles southwest of Hartford and 80 miles northeast of New York City.

The United States has experienced a number of mass shooting rampages this year, most recently in Oregon, where a gunman opened fire at a shopping mall on Tuesday, killing two people and then himself.

The deadliest attack came in July at a midnight screening of a Batman film in Colorado that killed 12 people and wounded 58.

This would be the deadliest elementary school shooting in U.S. history.

The worst U.S. high school shooting happened in 1999 when two students, Dylan Klebold and Eric Harris, went on a rampage at Columbine High School in Littleton, Colorado, killing 12 students and a teacher before turning their guns on themselves.

In 2007, 32 people were killed at Virginia Tech university in the deadliest act of gun violence in U.S. history.

The Anti-Defamation League expressed shock, disbelief and horror at the mass killing at a Newtown."We firmly believe that one way to limit the power of extremists and reduce violence in our communities is to enact tough, effective gun control legislation," Abraham H. Foxman, ADL National Director, and Gary Jones, ADL Connecticut Regional Director said in a statement.