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State police personnel led children from the school, following the shooting. (Courtesy of Shannon Hicks, / December 14, 2012 via Hartford Courant)

A 20-year-old gunman entered a Connecticut elementary school where his mother worked Friday morning and unleashed one of the worst massacres in American history,  leaving 27 people, including 20 children, dead before taking his own life.

The shooter, Adam Lanza, was dead, the AP reported, citing a source, and his older brother Ryan, of Hoboken, N.J., was questioned but was not involved, the AP said.  A mix-up by law enforcement led the media to initially believe that Ryan Lanza was the shooter, the AP reported.  Before the error was corrected, however, Lanza’s Facebook page was already under attack by Internet vigilantes.  Lanza, who works at Ernst & Young, posted on his phone on the way home from work, “IT WASN’T ME, I WAS AT WORK, IT WASN’T ME.”  His story was corroborated by swipe records at his Manhattan offices, ABC reported.

The shooter’s mother, Nancy Lanza, worked at the school as a teacher and was killed at the Newtown home she shared with her son before he set off on his deadly mission, the AP reports.

Eighteen children died at the school and two more were taken to area hospitals, where they were pronounced dead, Connecticut state police said, and six adults were killed. The principal is believed to have been one of the adults killed during the shooter’s spree.  The shooting was confined to one area of the school, and limited to two classrooms, Lt. Paul Vance said. The assistant principal, shot in the leg, survived and is being treated at Danbury Hospital.

“Evil visited this community today,” said Connecticut Gov. Dan Malloy.

The shooting happened at Sandy Hook Elementary School in the small Fairfield County town of Newton, about 75 miles northeast of New York City, about 9:40 a.m. Friday morning.

Three guns were found at the school —  a Glock and Sig Sauer pistols and a .223-caliber rifle, AP reports. The guns were purchased legally but registered to the shooter’s mother, reports said.

One of the guns found was a Bushmaster .223

A command post was set up at local firehouse. where children were reunited with their parents as others waited desperately for what would be horrific news. One classroom has ever single child unaccounted for.

In an age where it is almost impossible to eliminate electronic traces of one’s existence, Adam Lanza managed to leave few traces.  No Facebook page, no Twitter account, not even a photo in his high school yearbook, the NY Times reported.  Classmates described him as visibly pained in social interactions, a young man whose stark solitude was likely rooted in a personality disorder reported to be Asperger’s Syndrome.  Lanza was also left at home in the middle of a failing marriage after his older brother moved to attend college at Quinnipiaz University.  His parents were divorced in 2008.

Wiping away tears, President Obama choked up and paused as he addressed the nation at 3:15 Friday afternoon. Obama ordered flags lowered to half-staff. “Our hearts are broken today,” he said in moving comments. “The majority of those who died today are children.”

Obama also said the nation has to come together, take meaningful action after school shooting.  He will fly to Connecticut Saturday to meet with the community and state leaders.

Mourners console each other outside the Sandy Hook Fire Department (Cloe Poisson, Hartford Courant / December 14, 2012 )

“I heard seven loud booms and the gym teacher told us to go in the corner and we all huddled,” a student at the school told NBC Connecticut.

The school district’s superintendent has placed all buildings under lockdown, which has since been lifted.

A young girl is cradled following a fatal shooting at Sandy Hook Elementary School in Connecticut. (John Woike, Hartford Courant / December 14, 2012 )

Children at the school were evacuated. Some kids emerged from the school holding hands — some in tears as they were escorted from the building by teachers. Teachers sprang to action in the school as gunfire resonated in the hallways, ordering kids to huddle in the corners and locking the doors.

“Everybody was crying,” said Alexis Wasik, a third grade, according to CNN. “And I just heard the police officers yelling.

Said another woman outside the school, according to CNN:

“Why? Why?”

A mother rushes three children down Dickinson Drive after 28 people were shot and killed in a shooting Friday morning at Sandy Hook Elementary School ( Cloe Poisson, Hartford Courant / December 14, 2012 )

A woman talks to a state police officer at the scene of a shooting at Sandy Hook Elementary School in Newtown ( Cloe Poisson, Hartford Courant / December 14, 2012 )

Parents pick-up their children after a shooting at Sandy Hook

Stay with for updates to this developing story.