PARKLAND, Fla. — Daniel Medeiros was in a fourth-period English class, feeling worn down by a lesson on sonnets when the fire alarm went off. “Oh yeah,” he thought to himself. “Sweet.”

Moments earlier, Haeseung Lee had left his psychology class to go to the bathroom when he heard a succession of bangs. What he knows about firearms he learned from video games. But these sounded a lot like gunshots. He bolted for a classroom.

It was the beginning of six minutes of gunfire that would obliterate what was supposed to be a day like any other at Marjory Stoneman Douglas High School in Parkland, Fla. A troubled ex-student, Nikolas Cruz, had ridden to campus in an Uber with a legally purchased AR-15 semiautomatic rifle and a backpack full of ammunition. His lightning attack, between 2:21 p.m. and 2:27 p.m., would leave 17 people dead.

Second by second, lives at Stoneman Douglas were transformed, broken and stolen. Students locked themselves in closets. They considered fighting back, and considered their own deaths. And amid gunfire and chaos, they shot urgent text messages to their parents.