Students who had been at school with suspected gunman Nikolas Cruz, 19, said "everyone predicted" he could "do something" like the tragedy that unfolded on Wednesday afternoon, and that some children had been scared of him.
The sheriff's office also tweeted that the shooter was in custody and local TV footage showed officers putting a man into a police auto outside the school. "My triplets attended this school, and it's awful, just disgusting", Broward County sheriff Scott Israel confirmed said confirming the mass shooting inside the school.
As Cruz opened fire with an AR-15 rifle, Coach Feis, according to students, jumped in front of the bullets.
An unnamed student told the local WJXT news channel that students "joked" that Cruz would one day shoot up the school. A further two people died later while receiving medical treatment.
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"Our district is in a tremendous state of grief and sorrow", said Robert Runcie, superintendent of the school district in Parkland, about an hour's drive north of Miami.
According to a law enforcement source, the suspect is being investigated.
President Trump sent out his own Tweet offering his "prayers and condolences to the families of the victims of the bad Florida shooting".
Florida Governor Rick Scott said the shooting was "absolutely pure evil".
It has again renewed the debate surrounding gun control in the US.