Nikolas Cruz killed at least 17 people at Marjory Stoneman Douglas High School on Wednesday.
The 19-year-old former student flaunted pictures of firearms on his phone and posed with toy guns on social media.
Students previously "joked" that Cruz would shoot up the school, a student said.
Students at the Florida high school that lost at least 17 people to a deadly shooting on Wednesday "predicted" that Nikolas Cruz would "shoot up the school" one day, a student said.
Cruz, a 19-year-old former student, inflicted carnage with an AR-15 semiautomatic rifle on Wednesday afternoon.
Cruz, who was reportedly expelled from Marjory Stoneman Douglas High School for disciplinary reasons, flaunted pictures of firearms on his phone, and posed with toy guns on his Instagram account.
An unnamed student told the local WJXT news channel that students "joked" that Cruz would one day shoot up the school.
Cruz also familiarized himself with the school's layout and emergency procedures, so knew how the school would react, the student said.
The student said: "Honestly, a lot of people were saying it was going to be him. All kids joked around like that, saying that he was going to be the one to shoot up the school.
"It turns out everyone predicted it. That's crazy.
"He was on the third floor. He knows the school layout. He knows where everyone would be at.
"He's been with us through fire drills, he's prepared for this stuff."
Cruz is believed to have intentionally set off the school's fire alarms "so the kids would come pouring out of the classrooms into the hall," during the shooting, Florida Sen. Bill Nelson said on CNN
"There the carnage began," Nelson said.