The massacre at Sandy Hook Elementary School shook everyone in Newtown, Conn., including the first responders, who will be undergoing counseling. NBC's Anne Thompson reports.

Updated at 10:04 p.m. ET: A teacher's son, clad in black and carrying two 9mm pistols, rampaged through a Connecticut elementary school Friday, killing 20 small children and six adults, a tragedy President Barack Obama said had broken the hearts of America.

Undated photo confirmed by government officials to be Adam Lanza, who apparently killed himself after killing more than two dozen others, including 20 school children.

The gunman, identified as Adam Lanza, 20, was found dead at the scene of the slaughter, Sandy Hook Elementary School in Newtown, law enforcement officials said. The body of a woman believed to be his mother was found at their home in Newtown, authorities said.

Officials initially misidentified the shooter to NBC News as Lanza's brother, Ryan. But a senior official later said that Ryan was nowhere near the shooting, is not believed to be involved, and is cooperating with the investigation.

Ryan told police that Adam has a history of mental illness, according to the senior official. Yet the motive for the mass killing — the nation’s second-worst school shooting — was a mystery.

The weapons used in the attack were legally purchased and were registered to the gunman's mother, two law enforcement officials said. Two 9mm handguns were recovered inside the school. An AR-15-type rifle also was found at the scene, but there were conflicting reports Friday night whether it had been used in the shooting.

Police believe Lanza fatally shot his mother in the face, then drove to the hilltop school and unleashed a blizzard of bullets on children and staff in two rooms before apparently taking his own life.

"Evil visited this community today," Connecticut Gov. Dannel Malloy said Friday evening. "We are all in this together."

Adam Lanza, the man who allegedly committed one of the worst shootings in U.S. history, was 20 years old. He entered the Sandy Hook Elementary School carrying two handguns and hundreds of rounds of ammunition, police said. NBC's Pete Williams reports.

Some young survivors — ages 5 to 10 — described the terror of the shooting and a massive police response that included SWAT officers going room to room to search for victims as students huddled in classroom corners.

More coverage: NBCNewYork.com and NBCConnecticut.com

BreakingNews.com's coverage of the incident

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.”

The high death toll and the tender age of many victims sent shock waves all the way to the White House, where the flag was lowered to half-staff.

President Obama, his voice cracking at times, said he reacted to the tragedy first as a parent.

“Our hearts are broken today,'' he said. “The majority of those who died today were children. Beautiful little kids between the ages of 5 and 10 years old.”

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.

An elementary school student recalls the terrifying moments following sounds of shots fired at her Connecticut elementary school, saying "teachers told us to go in the corner so we all huddled."

Connecticut State Police Lt. Paul Vance said troopers fanned out across the school and searched “every door, every crack, every crevice” of the building.

Most of the bodies were found in two rooms in one section of the 600-student school, which goes up to the fourth grade. Late into the night on Friday, the bodies remained in the school during the investigation.

Two children were taken to Danbury Hospital, but they died. A third person was being treated at the hospital, which went into lockdown mode and cleared trauma rooms as doctors waited for an influx of survivors that never came.

After police finished searching the school and determined there was only one gunman, they led the children outside, telling them to close their eyes, apparently to avoid seeing anything gruesome.

At a staging area ringed by police vehicles that raced to the school from across the state, the dazed and crying kids were reunited with worried loved ones.

The shooting at Sandy Hook Elementary School that killed more than two dozen, 20 of them children, left the quiet community of Newtown, Conn., desperately trying to understand what happened. NBC's Kate Snow reports.

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." He went on to describe the anguish of waiting to find out if his son was a victim and then running to his child when he saw him.

“It was the greatest relief in my existence,” the father said. “I’m just happy that my kid’s OK.”

The FBI was assisting with the widening investigation, and authorities said there were many unanswered questions, including the motive.

“There is a great deal of search warrant activity…in and out of the state,” Vance said, without giving specifics.

Connecticut Gov. Dannel Malloy was meeting with grieving families.

“As you can imagine, the governor is horrified by what’s happened,” said aide Roy Occhiogrosso.

The death toll is the highest from a school shooting 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 of 27,000, 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.

Obama said Friday’s shooting, following the massacre at a movie theater in Aurora, Colo., and other murder sprees, showed the need for “meaningful action…regardless of the politics” to prevent more blood from being spilled.

New York City Mayor Bloomberg, who has been pushing for tougher gun laws, called for Washington to act immediately.

“Not even kindergarteners learning their A, B, Cs are safe,” he said. “We heard after Columbine that it was too soon to talk about gun laws. We heard it after Virginia Tech. After Tucson and Aurora and Oak Creek. And now we are hearing it again.”

Slideshow: Connecticut school massacre Michelle Mcloughlin / Reuters The second deadliest school shooting in U.S. history sent crying children spilling into the school parking lot as frightened parents waited for word on their loved ones. Launch slideshow

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