“It’s surreal,” Mr. Moskowitz said. “People don’t come to Parkland to open a business. They come to Parkland to raise a family. They come to Parkland to send their child to an A-rated school. They come to Parkland to live in the safest community in the county.”

A Day That Began With “Life”

Every morning at Stoneman Douglas High begins with an affirmation, which is read over the intercom. On Wednesday morning, the affirmation began with the word life.

“Life supports me in every way possible” were the first words students heard that day.

The announcement went on to speak of Valentine’s Day, and how everyone “deserves a safe and healthy relationship.”

The Astronomy Club announced that it was celebrating Black History Month on Thursday night with a showing of the movie “Hidden Figures.” But that event was postponed, as Stoneman Douglas canceled all school events and called in grief counselors.

The Drill That Wasn’t

Melissa Falkowski was teaching a journalism class when the fire alarm went off at about 2:30 p.m. It had been activated shortly before the shots began, perhaps an effort by the gunman to sow confusion.

And it was the second time the alarm went off Wednesday, after a fire alarm earlier in the day.

“Everyone is annoyed because we’d already had one today,” she said. “It’s unusual but it’s not unheard-of, because sometimes someone in culinary burns something and it might go off three times in a day.”

There also had been a drill a few weeks before: an active-shooter drill, the kind many schools now do.

Dutifully, Ms. Falkowski followed the fire drill protocol and filed her students out of the classroom to their appointed gathering spot. One of the school’s security officers told her: “No, it’s a code red. Go back.”

In geometry class, Gabriella Figueroa, 16, a junior, had been working on angle bisectors when the alarm sounded. As she neared the exit doors, she heard the first gunshots and ran back to class. “I was shaking and praying and saying, ‘God, please get me out of here,’” she said.

Ms. Falkowski took 20 students into a closet reserved for photo gear. Some of the students began to sob, but most stayed quiet, texting in the dark. It got so hot that they had to crack the door open periodically to get enough air.

Finally, after 35 minutes, they heard noises inside the classroom, and then a voice: “This is the police. Is anyone in here?”

The shooting had happened in a different building. None of them had heard gunfire.

Most of the 17 children and adults who died remained unidentified by the authorities as of Wednesday night. One of the complications, the sheriff said, was that the students’ identifications were still in their backpacks, which they had left behind in the havoc.