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."

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.

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.

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.