The gunman, identified as Adam Lanza, 20, was also found dead at Sandy Hook Elementary School in Newtown, a federal law enforcement official said. Lanza's mother is a kindergarten teacher at the hilltop school, and it's inside her classroom where most of the casualties were found, according to NBCNewYork.com.
Law enforcement officials initially told NBC News the gunman was Lanza's brother, Ryan. Another relative, who was not identified, was found shot to death at a home in Newtown, officials said.
Some young survivors -- all under age 10 -- described the terror of the shooting at Sandy Hook Elementary School, and a massive police response that included SWAT officers going room to room to search for victims as they huddled in classroom corners.
Robert Licata said his 6-year-old son was in class when the gunman burst in and shot the teacher.
"That's when my son grabbed a bunch of his friends and ran out the door," he told the Associated Press. "He was very brave."
One student told NBC Connecticut she was in the gym when she heard "seven loud booms."
"The gym teachers told us to go in the corner, so we all huddled," she said. "And I kept hearing these booming noises. And we all ... started crying.
"All the gym teachers told us to go into the office where no one could find us," she added. "So then a police officer came in and told us to run outside. So we did and we came in the firehouse."
Authorities in the small bedroom community 60 miles from New York City were alerted to the unfolding carnage by a 911 call around 9:30 a.m. and then reached out to state police and neighboring police departments for help.
Connecticut State Police Lt. Paul Vance said troopers who responded fanned out across the school and searched "every door, every crack, every crevice" of the building.
He said only that they found "several fatalities," including the shooter, at the scene.
The 600-student school goes up to the fourth grade. After the search was over, children and staff were escorted to a staging area outside.
Children said they were told to close their eyes as they were led outside, apparently to avoid seeing anything gruesome.
Over the next two hours, they were reunited with loved ones who rushed to the school, along with police cars and ambulances from all over Fairfield County.
Brenda Lebinski, mother of a third-grader, said she found a "horrendous" scene.
"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,'' she said, according to Reuters.
One parent picking up his 7-year-old son said the shooting was "the most terrifying moment a parent can imagine" and described the anguish of waiting to find out if his son was a victim and then running to his child.
"It was the greatest relief in my existence," the father said. "I'm just happy that my kid's OK."
Bracing for a large influx of wounded, Danbury Hospital went on lockdown and cleared four trauma rooms. It received only three patients, including a teacher shot in the foot, the Associated Press reported.
An official told NBCNewYork.com that two 9mm handguns were recovered from the scene. But The Associated Press said one of the guns was a .223-caliber rifle.
The FBI was on the scene, assisting with the investigation. "There is a great deal of search warrant activity...in and out of the state," Vance said, without giving specifics.
Connecticut Gov. Dannell Malloy was meeting with families near Sandy Hook.
"As you can imagine, the governor is horrified by what's happened," said aide Roy Occhiogrosso.
President Obama was told of the shooting at 10:30 a.m.
"I can just tell you that as a father, incidents like these weigh heavily on him, and I think everyone who has children and can imagine the enormous suffering that accompanies an event like this," White House press secretary Jay Carney said.
The death toll is the highest from a school shooting in U.S. history since a gunman killed 32 people at Virginia Tech in 2007. At Columbine High School in Littleton, Colorado, two teens killed 13 people and wounded 24 in 1999.
Parent Stephen Delgiadice, whose 8-year-old daughter was not hurt, said he never could have imagined such bloodshed in the quiet town, where the police force has only three detectives.
"It's alarming, especially in Newtown, Connecticut, which we always thought was the safest place in America," he told The Associated Press.