NEWTOWN, Conn. - Inside the four-bedroom Colonial set on a small rise, Nancy Lanza was already dead. But it was early yet, and it would be hours before her body was found - time enough for her son to unleash a slaughter.

For now, though, all seemed idyllic in this 300-year-old town under crystalline skies.

Adam Lanza - 20 years old, fascinated by computers and recalled by former classmates as painfully awkward - left the house in his mother's car and drove past fine old churches and towering trees. It was the holiday season, and lawns were decorated with lights and electric reindeer. It was just five miles from home to Sandy Hook Elementary, where hallways and classrooms rang with talk of Hanukkah and Christmas.

Inside the music room, a group of fourth graders were watching the movie The Nutcracker.

Theodore Varga and some other teachers were meeting. Their students, the oldest in the school, were in specialty classes like gym and music. The glow remained from the previous night's fourth-grade concert.

"It was a lovely day," Varga said. "Everybody was joyful and cheerful. We were ending the week on a high note."

The school appeared secure, its entrance monitored by closed-circuit camera and opened only when employees in the main office buzzed somebody in. But Lanza wasted no time, breaking through the window and opening the door.

And then, suddenly and unfathomably, gunshots rang out. "I can't even remember how many," Varga said.

Someone turned the loudspeaker on, so everyone in the building could hear what was happening in the office.

"You could hear the hysteria that was going on," Varga said. "Whoever did that saved a lot of people. Everyone in the school was listening to the terror that was transpiring."

The sounds reached a room where principal Dawn Hochsprung and school therapist Diane Day along with a school psychologist, other staff members, and a parent were gathered for a 9:30 meeting.

"We were there for about five minutes chatting, and we heard 'Pop! Pop!, Pop!' " Day told the Wall Street Journal. "I went under the table."

A custodian ran around, warning people there was a gunman, Varga said.

"He said, 'Guys! Get down! Hide!' " Varga said.

He survived.

At 9:30, Marci Benitez unlocked the door to Fun Kuts, the children's hair salon she and her husband run in the Sandy Hook neighborhood's small downtown, and prepared for the day.

Minutes later, the first police car streaked past, siren screaming. Then another. And another. And another.

Police radios crackled with first word of the shooting at 9:36, according to the New York Post.

"Sandy Hook School. Caller is indicating she thinks there's someone shooting in the building," a Newtown dispatcher radioed, according to a tape posted on the paper's website.

In the school, Hochsprung and school psychologist Mary Sherlach leaped out of their seats and ran out of the room. Hochsprung, 47, didn't think twice about confronting the gunman.

She died attempting to overtake Lanza, who was armed with two handguns and a .223-caliber Bushmaster rifle.

Sherlach also rushed to defend her students.

In a classroom, teacher Kaitlin Roig heard the shots and barricaded her 15 students into a tiny bathroom, sitting one of them on top of the toilet. She pulled a bookshelf across the door and locked it. She told the kids to be "absolutely quiet."

"I said: 'There are bad guys out there now. We need to wait for the good guys,' " she recounted to ABC News.

"The kids were being so good," she said. "They asked, 'Can we go see if anyone is out there?' 'I just want Christmas. I don't want to die. I just want to have Christmas.' I said, 'You're going to have Christmas and Hanukkah.' "

One pupil claimed to know karate. "It's OK. I'll lead the way out," the child said.

In the school library, clerk Maryann Jacob was working with a group of 18 fourth graders when she heard the commotion.

"We locked all our doors and then started hearing shooting," she said.

At first, she herded the children into a classroom within the library, but "when we realized the [classroom] door wouldn't lock, we had to crawl across the room into a storage room."

There, they locked the door and barricaded it with a filing cabinet. There happened to be materials for coloring, "so we set them up with paper and crayons."

In the gym, crying students huddled in a corner. One of them was 10-year-old Philip Makris.

"He said he heard a lot of loud noises and then screaming," said his mother, Melissa Makris. "Then the gym teachers immediately gathered the children in a corner and kept them safe."

Another girl who was in the gym recalled hearing "like, seven loud booms."

"The gym teacher told us to go in a corner, so we all huddled and I kept hearing these booming noises," the girl, who was not identified by name, told NBC News. "We all started - well, we didn't scream. We started crying, so all the gym teachers told us to go into the office where no one could find us."

An 8-year-old boy described how a teacher saved him.

"I saw some of the bullets going past the hall that I was right next to, and then a teacher pulled me into her classroom," said the boy, who was not identified by CBSNews.com.

Robert Licata said his 6-year-old son was in class when the gunman burst in and shot the teacher. "That's when my son grabbed a bunch of his friends and ran out the door," he said. "He was very brave. He waited for his friends."

He said the shooter didn't utter a word.

"The shooting appears to have stopped," the dispatcher radioed at 9:38, according to the Post. "There is silence at this time. The school is in lockdown."

And at 9:46, an anguished voice from the school: "I've got bodies here. Need ambulances."