[UPDATE 2:00 p.m. PST] An earlier report claiming the gunman's brother was found dead is being disputed by police.
[10 a.m.] A heavily armed gunman opened fire on school children and staff at a Connecticut elementary school on Friday, killing at least 28 people, including 20 children, in the latest in a series of shooting rampages that have tormented the United States this year.
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 20 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 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.
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.
BLOODIED CHILDREN EXIT SCHOOL
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."
President Barack Obama wipes a tear as he speaks about the shooting at Sandy Hook Elementary School in Newtown, Conn. during a press briefing at the White House on Dec. 14. Photo by Yuri Gripas/Reuters
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.
Obama remains committed to trying to renew a ban on assault weapons, Carney said.
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.
In another notorious school shooting outside of the United States, in 1996 a gunman opened fire in an elementary school in Dunblane, Scotland, and killed 16 children and an adult before killing himself.
Additional reporting by Dan Burns, Chris Francescani and Peter Rudegeair; Writing by Daniel Trotta and Jackie Frank