Story highlights Suspect was wearing black fatigues and a military vest

Law enforcement officials identify him as 20-year-old Adam Lanza

His mother was found dead at a Newtown residence

The nation and the world react with shock

Dressed in black fatigues and a military vest, a heavily armed man walked into a Connecticut elementary school Friday and opened fire, shattering the quiet of this southern New England town and leaving the nation reeling at the number of young lives lost.

Within minutes, 26 people were dead at Sandy Hook Elementary School -- 20 of them children. Among the six adults killed were Dawn Hochsprung, the school's beloved principal, and school psychologist Mary Sherlach.

The shooter, identified by three law enforcement officials as 20-year-old Adam Lanza, also was killed, apparently by his own hand. Separately, his mother's body was found at a Newtown residence.

"Stuff like this does not happen in Newtown," a tight-knit community of about 27,000 just outside Danbury, said Renee Burn, a local teacher at another school in town. In the past 10 years, only one homicide had previously been reported.

With the death toll at 26, the Newtown shooting is the second-deadliest school shooting in U.S. history, behind only the 2007 shooting at Virginia Tech that left 32 people dead.

Photos: Photos: Newtown school shooting Photos: Photos: Newtown school shooting Newtown school shooting – Children in Newtown, Connecticut, return to school on December 18, 2012, four days after the shooting at Sandy Hook Elementary School. Hide Caption 1 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police officers search outside St. Rose of Lima Roman Catholic Church in Newtown, Connecticut, on Tuesday, December 16, after a threat prompted authorities to evacuate the building. Investigators found nothing to substantiate the reported threat, a police official said, declining to provide additional details. The church held Sunday services following the mass shooting at Sandy Hook Elementary School in Newtown. Hide Caption 2 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police officers walk out of St. Rose of Lima Roman Catholic Church after the Newtown church received a threat December 16. Hide Caption 3 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Firefighters attach black bunting to a fire truck as a memorial at the fire station down the street from the Sandy Hook Elementary School in Newtown, Connecticut, on Saturday, December 15. Hide Caption 4 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut Chief Medical Examiner H. Wayne Carver II talks to the media about the elementary school shooting during a press conference at Treadwell Memorial Park on December 15. Hide Caption 5 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Zulma Sein is hugged by a family member outside of the entrance to the Sandy Hook School on Saturday. Hide Caption 6 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Police officers keep guard at the entrance to the street leading to the Sandy Hook Elementary School on Saturday, December 15. Hide Caption 7 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police Lt. Paul Vance addresses the press on December 15. Hide Caption 8 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Police officers stand at the entrance to the street leading to the Sandy Hook Elementary School on December 15. Hide Caption 9 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Corinne McLaughlin, a student at the University of Hartford, bows her head during a candlelight vigil at Hartford, Connecticut's Bushnell Park on Friday, December 14, honoring the students and teachers who died at Sandy Hook Elementary School in nearby Newtown earlier in the day. Hide Caption 10 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Distraught people leave the fire station after hearing news of their loved ones from officials on Friday. Hide Caption 11 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Emergency workers stand in front of the Sandy Hook Elementary School in Newtown. Hide Caption 12 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A child and her mother leave a staging area outside Sandy Hook Elementary School in Newtown, Connecticut, on December 14. Hide Caption 13 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Members of the media converge on December 14 in front of an apartment at 1313 Grand Street in Hoboken, New Jersey. The apartment is believed to be connected to the Connecticut elementary school shooting. Hide Caption 14 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Faisal Ali, right, of Colorado Springs, Colorado, joins other people outside the White House on December 14 to participate in a candlelight vigil to remember the victims of the Sandy Hook Elementary School shooting. Hide Caption 15 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police spokesman Lt. J. Paul Vance, center, briefs the media on the elementary school shootings during a press conference at Treadwell Memorial Park on December 14 in Newtown. Hide Caption 16 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People weep and embrace near Sandy Hook Elementary School on Friday, December 14. Hide Caption 17 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A woman leans on a man as she weeps near Sandy Hook Elementary School on December 14. Hide Caption 18 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – President Barack Obama wipes a tear as he speaks about the shooting at Sandy Hook Elementary School during a press briefing at the White House on December 14. Hide Caption 19 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A woman weeps near the site of a shooting at Sandy Hook Elementary School on December 14. Hide Caption 20 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A woman weeps near Sandy Hook Elementary School on December 14. Hide Caption 21 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People comfort each other near Sandy Hook Elementary School on December 14. Hide Caption 22 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A man takes in the scene near Sandy Hook Elementary School on December 14. Hide Caption 23 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A young girl is given a blanket after being evacuated from Sandy Hook Elementary School on December 14. Hide Caption 24 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – State police personnel lead children from the school. Hide Caption 25 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Children wait outside Sandy Hook Elementary School in Newtown, Connecticut, after the shooting. Hide Caption 26 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A boy weeps at Reed Intermediate School after getting news of the shooting at Sandy Hook Elementary School on December 14. Hide Caption 27 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – FBI SWAT team members walk along Dickinson Drive near Sandy Hook Elementary School on December 14. Hide Caption 28 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – An aerial view of Sandy Hook Elementary School in Newtown, Connecticut on December 14. Hide Caption 29 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Troopers arrive on the scene outside Sandy Hook Elementary School on December 14. Hide Caption 30 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A Connecticut State Police officer runs with a shotgun at the Sandy Hook Elementary School in Newtown on December 14. Hide Caption 31 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Police patrol the streets around Sandy Hook Elementary School on December 14. Hide Caption 32 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People try to deal with the shock of the attack outside Sandy Hook Elementary School on December 14. Hide Caption 33 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police secure the scene of the shooting on December 14. Hide Caption 34 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People embrace outside Sandy Hook Elementary School on December 14. Hide Caption 35 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A man escorts his son away from Sandy Hook Elementary School on December 14. Hide Caption 36 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People take in the news outside Sandy Hook Elementary School on December 14. Hide Caption 37 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People line up to enter Newtown Methodist Church near the the scene of the shooting on December 14. Hide Caption 38 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A woman speaks with a Connecticut state trooper outside Sandy Hook Elementary School on December 14. Hide Caption 39 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – The streets around Sandy Hook Elementary are packed with first responders and other vehicles. Hide Caption 40 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A view of the scene at Sandy Hook Elementary School after the shooting. Hide Caption 41 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A young boy is comforted outside Sandy Hook Elementary School on December 14. Hide Caption 42 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People embrace each other on December 14. Hide Caption 43 of 43

JUST WATCHED Suspected shooter's brother questioned Replay More Videos ... MUST WATCH Suspected shooter's brother questioned 01:09

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"Evil visited this community today," Connecticut Gov. Dan Malloy said of Friday's massacre.

Young students described being ushered into bathrooms and closets by teachers as the first shots rang out. Janet Vollmer, a kindergarten teacher, locked her classroom doors, covered the windows and read a story to her 19 students to keep them calm.

Third-grader Alexis Wasik said police and teachers barged into her classroom and told students to hide in the corner.

"Everybody was crying," she said. "And I just heard the police officers yelling."

One parent who was in the school at the time of the shooting said she heard a "pop, pop, pop," sound around 9:30 a.m. In the room with her were Hochsprung, the vice principal and Sherlach. All three left the room and went into the hall to see what was happening. The parent ducked under the table and called 911.

"I cowered," she told CNN's Meredith Artley. The shooter "must have shot a hundred rounds."

Responding police officers helped evacuate the children, telling them to hold hands and keep their eyes closed to the carnage as they exited the building.

As reports of the shooting surfaced, frantic parents descended on a nearby firehouse where the children had been taken.

"Why? Why?" one woman wailed as she walked up a wooded roadway leading from the school.

Police declined to speculate on a motive Friday evening, citing the ongoing investigation. Lanza had no known criminal record, a law enforcement official said.

Alex Israel, a former classmate of Lanza's, described him as "quiet" and "reserved."

"You could definitely tell he was a genius," Israel told CNN, adding she hadn't talked with him since middle school. "He was really quiet, he kept to himself."

Three weapons were recovered from the school: a semi-automatic .223 Bushmaster rifle, a Glock and a Sig Sauer, a law enforcement official familiar with the investigation said. The weapons were legally purchased by Lanza's mother, the official said. A fourth weapon, a shotgun, was recovered from a car in the school parking lot, police said.

The official said Lanza died of an apparent self-inflicted gunshot wound. Lt. J. Paul Vance of the Connecticut State Police said that the medical examiner will determine the cause of death for the gunman, though he noted that police never discharged their weapons.

In addition to the killings at Sandy Hook, another adult was found dead at a second location in Newtown, Vance said. A law enforcement source with detailed knowledge of the investigation identified that person as Lanza's mother, Nancy.

Despite earlier reports that said she was a teacher, Nancy Lanza was not a teacher at the school where the killings took place, said Janet Vollmer, a kindergarten teacher at Sandy Hook Elementary School.

Meanwhile, authorities in Hoboken, New Jersey, were questioning Ryan Lanza, the suspect's older brother, law enforcement sources said, though they did not label him a suspect. Lanza's father, who lives in Connecticut, was similarly questioned, one of the law enforcement officials said.

Earlier, investigators had identified Ryan Lanza as the shooter. It was not clear what caused the confusion among investigators.

The fact that so many of those killed were young children -- Sandy Hook serves students in kindergarten through fourth grade -- touched a nerve across the world Friday, from U.S. President Barack Obama and other world leaders expressing grief to regular folks reacting with horror on social media as the news unfolded. The overwhelming sentiment: Hug your kids closely.

Obama, a father of two girls, wiped away tears while delivering a statement about the shooting, saying, "Our hearts are broken today."

"The majority of those who died today were children. Beautiful little kids between the ages of 5 and 10 years old," he said.

The bodies of the young victims remained where they fell Friday night, as authorities worked to positively identify them.

Flags were ordered to fly at half-staff nationwide in tribute to the victims, and candlelight vigils were planned across the country as Americans came together to try to comprehend the tragedy.

At a vigil in Newtown, just a mile away from the school, mourners spilled from St. Rose of Lima Roman Catholic Church, which was filled to capacity.

"This is most definitely the worst thing we've experienced in town," Newtown Police Lt. George Sinko said.

Parents, meanwhile, struggled to come up with answers for their children who were among those safely evacuated Friday.

Aimee Seaver's daughter is a first-grader at Sandy Hook.

"It's a very rough night here," she told "AC 360." "When your first-grader goes to bed and says, 'Mommy, is anyone from my class last year -- are they all OK?' and you look at them and say, 'I'm not really sure,' it's a rough night to tell that to your 7-year-old."

It is not yet known how the gunman entered the school.

Hochsprung, the slain principal, had recently installed a new security system to ensure student safety.

Under the new system, every visitor was required to ring a doorbell at the front entrance after the doors locked at 9:30 a.m. and report to the main office to sign in.

Police began receiving reports of shots fired around 9:40 a.m. Friday.

In 1999, at Columbine High School in Littleton, Colorado, two students shot 13 people to death before killing themselves.