PARKLAND, Fla. (CN) – The death toll in a mass shooting at a South Florida high school rose to 17 late Wednesday, and authorities have arrested an expelled student whom they say posted disturbing imagery on social media prior to the attack.
The shooter, identified as 19-year-old Nikolas Cruz, opened fire at Marjory Stoneman Douglas High School in Parkland, Florida, on Wednesday, shortly before the school day came to a close, police say. He was armed with an AR-15 rifle and "countless magazines," Broward County Sheriff Scott Israel said at an evening press conference.
Hundreds of students dashed into the streets outside the high school before 3 p.m. SWAT team members swarmed the campus as armored vehicles stood in front of a school entrance.
One student told local media that he saw his friend shot in the head.
Another youth, a junior at the school, told Fox 29 that she hid in a closet during the attack.
"I heard people screaming, 'Help me,'" she said.
When she made a break for an exit, she saw that the ground was stained with blood.
"We were running down the stairs, and I was trying not to slip. There was blood all over the floor," the student said.
According to Sheriff Israel, Cruz was a former student who had been expelled "for disciplinary reasons."
"[Cruz] is in custody. And we have already begun to dissect his websites and social media," Israel said at the press conference.
The sheriff said the material Cruz had posted on the internet prior to the shooting was alarming. Preliminary reports indicate that Cruz had been brandishing guns and knives on social media.
"Some of the things that come to mind are very, very disturbing," Israel said.
Jim Gard, a teacher at the school, told the Miami Herald that Cruz was a student in his class. He told the newspaper "there were problems with [Cruz] last year threatening students."
Cruz was captured by police off campus at roughly 4 p.m. and taken to an area hospital, according to law enforcement reports. He was later seen being escorted by police into a booking area, still wearing a hospital gown.
Florida Gov. Rick Scott said this afternoon that he is traveling to Broward County to be briefed by emergency management officials.
The sheriff meanwhile urged the community to reach out to police about disturbing content on the internet, to prevent future tragedy. He noted that three of his children once attended the high school.
"If anybody has any indicator that someone is going through a behavioral change, or on their social media, that there are disturbing photos, perhaps bombs or firearms, please make sure law enforcement knows about it," Israel said at the evening press conference.
The week before the shooting, Parkland had been named the third safest city in the state by the National Council for Home Safety and Security.
Broward County school superintendent Robert Runcie said that the community is "experiencing the worst of humanity."
"This is a day we prayed would never be here in Broward County," Runcie said.
Israel said at an initial briefing, "It's catastrophic. There really are no words."
Read the Top 8
Sign up for the Top 8, a roundup of the day's top stories delivered directly to your inbox Monday through Friday.