A gunman who opened fire at a South Florida high school Wednesday afternoon, killing at least 17 people and injuring dozens more, is now in custody.

The death and injury count is expected to rise as new reports come in.

The shooting began around dismissal time, when the gunman — who has been identified as a former student named Nicolas de Jesus Cruz — opened fire in a freshman building at the Marjory Stoneman Douglas High School in Parkland. Live video from the scene showed the school on lockdown, surrounded by SWAT teams and tanks, as students evacuated the campus, some fleeing the scene in small groups, and others walking calmly out in larger groups, holding their hands over their heads. A triage scene was erected across the street from the school, where medics are treating victims under a large red tent.

The shooting began shortly before dismissal, the school said, and students initially thought the evacuation was a fire drill. But it quickly became clear something was amiss.

“As soon as the fire alarm got pulled, and we started evacuating, I heard five pops,” one former student in the building told WSVN, telling the network that there was initially confusion because of a fire drill in the school that took place earlier in the day. “I knew it was more than a drill.”

Another student told the network that when he first heard the shots, he thought it might be balloons popping because of all the balloons in the building for Valentine’s Day.

“He’s a troubled kid,” who had a lot of guns and showed other students pictures of them on his phone, a student who knew Cruz told WSVN. “He knows the layout of the school, he’s been in the [shooting] drills multiple times, he knows where to go.”

The student's recollections were echoed by Jim Gard, a math teacher at the school, who told the Miami Herald that Cruz was a former student of his who had been banned from bringing backpacks to school because of his troubling behavior.

“We were told last year that he wasn’t allowed on campus with a backpack on him,” Gard said. “There were problems with him last year threatening students, and I guess he was asked to leave campus.”