PARKLAND, Florida – At least 17 people were dead after a 19-year-old former student opened fire at a South Florida high school on Wednesday afternoon, officials said.

The suspect was identified as Nikolas Cruz, a former student who had been expelled from Marjory Stoneman Douglas High School in Parkland for disciplinary reasons, Broward County Sheriff Scott Israel said. He said at least 14 other people were injured in addition to the 17 people killed.

Officials believe the gunman had one AR-15-style semiautomatic rifle and multiple magazines, but it was unclear whether he had any other weapons. The gunfire began outside the school and continued inside, where 12 of the victims were killed, Israel said.

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Federal and local authorities told NBC News that there was no indication that the gunman had an accomplice or accomplices.

What we know so far:

  • 17 people were killed, according to officials, both inside and outside Marjory Stoneman Douglas High School in Parkland, Florida
  • The shooting started around 2:30 p.m. ET on Wednesday as the fire alarm sounded
  • A suspect, Nikolaus Cruz, 19, a former Douglas High student believed to have been armed with a semiautomatic rifle and multiple magazines, is in custody

Authorities initially spelled Cruz’s first name as “Nikolaus” but later corrected the spelling. He was taken into custody off campus about an hour after he “committed this horrific, detestable act,” said Israel, who said investigators were reviewing social media postings that he described as “very disturbing.”

Eddie Bonilla, a senior, told NBC Miami that Cruz was “a little bit off” and “troubled.”

Cruz used to show off his guns, brag about shooting them “for fun” and “threatened to bring the guns to school multiple times,” he said, adding that students “threw jokes around that he’d be the one to shoot up the school.”

Photos: Florida school shooting leaves families distraught

Sebastian Toala, another senior, told the station: “I never really got close to him because I always had a feeling there was something wrong.”

Parkland, in north Broward County, is about 30 miles northwest of Fort Lauderdale. The shooting on the sprawling campus happened despite the presence of police officers at the school.

Broward County schools Superintendent Robert Runcie said at last two police cars were typically on campus “on a daily basis.”

While students filed out of the school with their hands up, heavily armed SWAT team members conducted a class-by-class search to make sure there were “no other shooters” — and to retrieve any bodies, he said.

Israel, who said his triplets once attended that school, urged worried parents to head to a nearby Marriott hotel to collect their children.

“This is a terrible day for Broward County, the state of Florida, the United States,” he said. “There really are no words.”

The first sign that something awful was happening came around 2:30 p.m., not long before classes were supposed to have been dismissed, when authorities were called to respond to an active shooter.

For more than an hour, the school was at the mercy of a gunman on the loose.

“He was outside and inside the school,” Israel said.

Just after 4 p.m., the Broward County Sheriff’s Office announced on Twitter that the suspect had been apprehended.

Not long after, stunned survivors began sharing their accounts of what happened.

“I was on the first floor of the building where the shots happened first,” freshman Jason Snytte told NBC News. “I was in the classroom right next to the outside, the closest classroom to it. It started there. I heard six or seven shots. Our door was open. I ran and closed it, and everybody ran into a corner.”

President Donald Trump was briefed and was monitoring the situation, White House Press Secretary Sarah Huckabee Sanders said.