By Susan Candiotti and Sarah Aarthun
NEWTOWN, Connecticut (CNN) -- 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.
"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.
"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.
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.
Three weapons were recovered from the school: a semi-automatic .223 Bushmaster found in a car in the school parking lot, and a Glock and a Sig Sauer found with Lanza's body, a law enforcement official familiar with the investigation said. The weapons were legally purchased by Lanza's mother, the official 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, who was found dead at a Newtown residence. She was a teacher at Sandy Hook, sources said.
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.
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.
"This is most definitely the worst thing we've experienced in town," Newtown Police Lt. George Sinko said.
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.
CNN's Susan Candiotti reported from Newtown, Connecticut; Sarah Aarthun from Atlanta. CNN's Michael Pearson, Meredith Artley, John King, David Ariosto, Ashleigh Banfield, Joe Johns and Terry Frieden contributed to this report.