(CNN) -- The 5-year-olds in Janet Vollmer's kindergarten class heard the noise: Pop. Pop. Pop.
What was that noise? They looked to Vollmer for answers.
They were too young to understand what it meant when they heard the gunfire Friday outside their classroom door at Sandy Hook Elementary School in Newtown, Connecticut.
They never heard of Columbine. They didn't know anything about Virginia Tech. These children, at that tender age, couldn't comprehend the kind of carnage created by a mass shooting -- the same kind of shooting that by the end of the day would make their school the scene of the second worst school shooting in U.S. history.
Outside Vollmer's classroom, a heavily armed gunman wearing black fatigues and a military vest was taking aim at children and educators.
In a few minutes, 20 children and six educators were dead. Eighteen of the pupils -- between the ages of 5 and 10 -- died where they fell, investigators say. Two more were pronounced dead at an area hospital.
Inside the classroom, Vollmer's job -- as it has been nearly every day for 18 years -- was to keep the children calm, focused on the task at hand.
"You could hear what sounded like pops, gunshots," she told CNN late Friday.
The children had been through emergency lockdown drills at the schools. So when Vollmer locked the doors and put the blinds down they knew what to do, "go over in the safe area" in the back of the classroom.
There, Vollmer read to the children.
Still the children knew something was not right. "It didn't seem a natural thing to them," she said.
When they asked questions about what was happening, Vollmer and her teaching aides told them: "We're not really sure, but we're going to be safe, because we're sitting over here and we're all together."
First-grade teacher Kaitlin Roig heard what sounded like the rapid firing of an assault rifle.
"I knew something was wrong," she told ABC "World News" anchor Diane Sawyer.
She herded the 14 children, ages 6 and 7, into the class bathroom. She helped some climb onto the toilet so they could all fit in the tiny room. Then she locked the door.
"I just told them we have to be absolutely quiet," Roig told Sawyer.
As the minutes ticked by, the children asked Roig if the could "go see if anyone is out there."
No, she told them.
"If they started crying, I would take their face and tell them, 'It's going to be OK.' I wanted that to be the last thing they heard, not the gunfire in the hall."
In a nearby classroom, 8-year-old Alexis Wasik didn't know what was going on.
She, too, heard the shots. But it wasn't until she heard the sirens wail that she and her classmates put it together: A shooting at the school.
"Everybody was crying," she told CNN.
Alexis and her classmates huddled together in the back of the classroom.
"We heard an ambulance and police officer come and everyone was a little scared, crying, and I felt actually a little sick and like I was going to throw up," she later told ABC's Sawyer.
"Kids were crying, not really like screaming, but they were all huddling together."
Back in Vollmer's classroom, she was concentrating on her job "to keep them safe."
There was no announcement over the school's loudspeaker to announce an emergency, a routine occurrence during emergency drills.
"My instinct was that it wasn't good," Vollmer said.
Soon police officers were banging on Vollmer's classroom door.
The kindergartners were told to line up and cover their eyes as they were led by police out of the school to a nearby firehouse, Vollmer said.
Police also evacuated Roig and her students to the firehouse.
Somewhere in the school building, authorities later said, was the gunman's body. Police believe he committed suicide, turning a gun on himself.
Also in the building, the bodies of classmates and faculty.
As reports of the shooting made their way around town, 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.
Inside the firehouse, Vollmer's kindergartners were beginning to understand something terrible had happened.
"They saw other people upset," Vollmer said. "We just held them close until their parents came."
Alexis also ended up at the firehouse. There, she was reunited with her mother.
"It just doesn't seem real," Alexis' mother told WABC. "It feels like a nightmare. You drop your kids at school, hugs and kisses, have a good day, I'll see you later and see you at the end of the day and you never know, in 20 minutes from now what's going to happen."
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CNN's Greg Botelho and Jason Hanna contributed to this report.