At least 27 people, including 18 children, were killed on Friday when at least one shooter opened fire at an elementary school in Newtown, Connecticut, CBS News reported, citing unnamed officials.
If confirmed, it would be one of the worst mass shootings in U.S. history. The holiday season tragedy follows a series of shooting rampages in the United States this year that have killed multiple victims, and it 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.
There were unconfirmed reports of a second shooter.
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.
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 NBC Connecticut. The mayor of Danbury, Mark Boughton, told MSNBC: "They are very serious injuries."
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.
One child was carried from Sandy Hook Elementary School by a police officer, and the child appeared to have been wounded, the town's weekly newspaper, the Newtown Bee, said on its website.
Connecticut State Police said its officers were at the scene with local police but provided no additional details. The emergency call to police occurred at 9:41 a.m., state police said.
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.
Sandy Hook is one of four elementary schools in the district.
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.