Who is alleged school shooter Nikolas Cruz?

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Nikolas Cruz (Broward County Sheriff's Office)

Nikolas Cruz, the suspect in a mass shooting at a South Florida high school Wednesday, has been booked on 17 premeditated murder counts.

Police say he was equipped with a rifle, gas mask, smoke grenades, and ammunition when he went to the school. They say he pulled the fire alarm to send students into the hallways. He then opened fire, sending hundreds of students fleeing into the streets and others hiding in the school for more than an hour.

Cruz, 19, is a former student at Marjory Stoneman Douglas High School in Parkland, Broward County Sheriff Scott Israel said. The school had expelled him for disciplinary reasons.

SWAT team members captured Cruz off-campus after the shooting.

The superintendent of the school district said Cruz didn't make any specific threats ahead of the shooting. But some teachers said Cruz had demonstrated some warning signs, Fox News reported.

An Instagram page apparently belonging to Cruz showed pictures of guns and knives.

The sheriff's office said the gunman had an AR-15-style semiautomatic rifle and multiple magazines. A law enforcement official told the Associated Press that Cruz had legally purchased an AR-15 rifle about a year ago.

The leader of a nationalist militia group said Cruz was a member and participated in paramilitary drills in Tallahassee. Jordan Jereb told the Associated Press that his group wants Florida to become its own white ethnostate but added that he did not know Cruz personally.

The FBI has set up a contact form for information, photos, and videos that could be helpful in the investigation.

14 wounded survivors remained hospitalized on Thursday.

Wednesday's attack was the nation's deadliest school shooting since the Newtown, Connecticut, attack just over five years ago.