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.

"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 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.