19-year-old Nikolas de Jesus Cruz - a former student at the Marjory Stoneman Douglas High School in Parkland - reportedly entered the school armed with an assault rifle and began shooting just after 2pm on Valentine's Day.
Broward Sheriff Scott Israel confirmed that 17 people had been killed as a result.
'It's catastrophic. There really are no words,' he said.
Cruz managed to make it off campus before he was cornered and taken into custody.
Matthew Walker, a 17-year-old student from the school, told WFOR-TV that Cruz constantly posted on social media about weapons and that they 'knew it would be him'.
'A lot of kids threw jokes around saying that he was going to be the one to shoot up the school,' he said.
'He was going class to class just shooting at random kids. Everything he posts (on social media) is about weapons. It's sick.'
Alarmingly, Math teacher Jim Gard, who taught Cruz last year, told the Miami Herald that Cruz had been identified as a potential threat to students in the past.
'We were told last year that he wasn’t allowed on campus with a backpack on him,' he said.
'There were problems with him last year threatening students and I guess he was asked to leave campus.'
Another student, Nicholas Coke, said he was sitting in class when the fire alarm went off before he heard sounds of loud pops.
As he ran, he said people started jumping fences while others stayed and hid in classrooms.
'I wasn't going to stick around and find out what was going on,' he said.
Meanwhile, the devastated school superintendent, Robert Runcie, spoke at a press conference near the school.
'It’s a day that you pray every day when you get up that you will never have to see. It is in front of us. I ask the community for prayers and their support for the children and their families,' he said.
'We received no warning... Potentially there could have been signs out there. But we didn’t have any warning or phone calls or threats that were made.'