In a student’s Snapchat video, the sound of gunfire can be heard inside a classroom as students cried and crouched under their desks.
"I was just praying. Praying to God it was not me," said Trayvon Telfair, a sophomore at the school.
Telfair said he was in the freshman building and was feet away from the gunfire.
"One of the bullets came through the window and I just looked and everybody was in the room screaming. The teacher was telling them to be quiet," he said.
He was one of the dozens of students led out of the building by armed officers as they walked out with their hands above their heads.
Panic ensued as the school was put on lockdown.
Some students were led to safety, but other students had to remain barricaded in classrooms until police could reach them.
One student described waiting near the body of a teacher who had been shot.
“We saw his body for like 30 minutes. We were just praying and crying, and then the police came and we just got out,” said student Bruna Oliveda.
As students ran from the school, they described the chaos inside.
“We heard the pops and we were in the building at the back of the school,” said a student, who was not identified.
As students were led out of the school, some were told to keep their hands on the shoulders of the students in front of them and not to look to the side.
They were told to put their backpacks in a pile and move away.
Student Christina Vega said she never wants to step on campus again.
“I can’t go up the stairs. There were just trails of blood,” she said. “Our teacher, right in the corner, you can just see the bullet and like, blood on the wall. And then this kid on the other side."
The Broward County Sheriff's Office said the 17 victims include students and adults.
Investigators said the suspected gunman is former student 19-year-old Nikolas Cruz.
He was taken into custody about 2 miles from the school.
School officials said Cruz had been expelled from the school for disciplinary reasons.
Investigators said Cruz had at least one rifle and numerous magazines of ammunition.
There is no known motive for the shooting.
School was canceled for the remainder of the week and multiple grief counselors will be available, officials said.
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Florida Gov. Rick Scott told reporters Wednesday evening he can't imagine what the families of the victims are going through. He also said he would be visiting hospitalized survivors.
Sheriff Scott Israel of Broward County also said at the news conference that families of the dead are being notified. He said 12 of the dead have been identified. He says not all victims were carrying identification and thus couldn't be quickly identified.
Florida Attorney General Pam Bondi says the state will cover funeral expenses for the victims and counseling for survivors.