First, he killed his mother.
Nancy Lanza's body was found later at their home in Newtown - after the carnage at Sandy Hook Elementary School.
Nobody knows why 20-year-old Adam Lanza killed his mother, why he then took her guns to the school and murdered 20 children and six adults.
But on Friday he drove his mother's car through this 300-year-old town and arrived at a local school. Somehow, he got past a security door to a place where children should have been safe from harm.
Theodore Varga and other fourth-grade teachers were meeting. 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."
And then, suddenly and unfathomably, gunshots rang out. "I can't even remember how many," he said.
The fourth-graders, the oldest kids in the school, were in specialty classes like gym and music. There was no lock on the meeting room door, so the teachers had to think about how to escape, knowing that their students were with other teachers.
Someone turned the loudspeaker on, so everyone 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."
Gathered in another room for a meeting were principal Dawn Hochsprung and Diane Day, a school therapist, along with a school psychologist, other staff members and a parent. They were meeting to discuss a second-grader.
"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."
But Hochsprung and the psychologist leaped out of their seats and ran out of the room, Day recalled. "They didn't think twice about confronting or seeing what was going on," she said. Hochsprung was killed, and the psychologist was believed to have been killed as well.
A custodian ran around, warning people there was a gunman, Varga said.
"He said, 'Guys! Get down! Hide!"' Varga said. "So he was actually a hero."
Did he survive? The teacher did not know.
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 a first-grade classroom, teacher Kaitlin Roig heard the shots. She immediately 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 told 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 student claimed to know karate. "It's OK. I'll lead the way out," the student said.
In the gym, crying fourth-graders 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 am, according to the Post. "There is silence at this time. The school is in lockdown."
And at 9:46 am, an anguished voice from the school: "I've got bodies here. Need ambulances."
Carefully, police searched room to room, removing children and staff from harm's way. They found Adam Lanza, dead by his own hand after shooting up two classrooms; no officer fired a gun.