Nikolas Cruz, 19, suspected of killing 27 people after opening fire at Marjory Stoneman Douglas High School in Parkland, Florida, was reportedly obsessed with guns.
The alleged shooter had even "joked" about taking out his schoolmates on several occasions, one student said.
"He played around with the joke multiple times," the teen said. "You know, he knows the layout of the school. He can actually go ahead and he can pinpoint where all the students would be. He's been through the drills multiple times."
Law enforcement sources told the New York Post that Cruz was a member of the US Army Junior Reserve Officers' Training Corps program, otherwise known as JROTC.
Cruz had been banned from carrying a backpack on campus due to his alleged issues, before he was ultimate expelled for his behavior.
He was also reportedly wearing a gas mask and carrying smoke grenades during the shooting.
According to the Broward Sheriff's Office, Cruz was taken into custody as several victims were transported to Broward North in Deerfield Beach and Broward Health Medical Center.
Seventeen people were killed during the school shooting. Cruz reportedly began shooting around 2:30 p.m. on Wednesday, minutes before the school day's dismissal. Using an AR-15 rifle, Cruz reportedly pulled the school's fire alarm then opened fire as kids began filing out of classrooms.
"We all thought it was a fire drill because we had one previously today," a student told FOX 6. "And we thought it was, so no one was that nervous, but then word started going around that it was shots and not just, like, something else, everyone just started running towards the canal."
One student managed to live tweet the shooting, while others posted videos to social media from inside classrooms. Police and SWAT arrived on the scene and helped many of the students out of the building, while others quickly fled on foot without being seen by Cruz, who was detained outside the school.
Cristina Vega, a student, told 7 News Miami that returning to the school will be extremely difficult.
"I don't want to come back to this school," she said. "I can't go up the stairs because up the stairs there was just trails of blood. Our teacher, right there in the corner.You just see the bullet and the blood on the wall."