PARKLAND, Fla. (CBSNewYork/AP) — A former student opened fire with a semi-automatic rifle at a Florida high school Wednesday, killing at least 17 people and sending hundreds of students fleeing into the streets in the nation’s deadliest school shooting since a gunman attacked an elementary school in Newtown, Connecticut in 2012.

The shooting occurred Wednesday afternoon at Marjory Stoneman Douglas High School in Parkland.

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Broward County Sheriff Scott Israel said that 12 of the victims had been identified, but their names would not be released until families were identified.

Israel said 12 of the victims were found inside the school, two more were found outside of the school, another was found on the street, and two more died under medical treatment.

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Several wounded were seen on stretchers being loaded into waiting ambulances, others were taken to a makeshift triage area with red medical tents.

As students walked out of the building single file with their hands up or on their heads SWAT teams made their way in armed with long guns and tactical gear, searching for the gunman. Others searched a nearby parking lot with K9s.

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The shooter, who was equipped with a gas mask and smoke grenades, set off a fire alarm to draw students out of classrooms shortly before the day ended at one of the state’s largest schools, officials said.

Authorities offered no immediate details on the 19-year-old suspect or any possible motive, except to say that he had been kicked out of Marjory Stoneman Douglas High School, which has about 3,000 students.

Students who knew the shooter, identified as Nikolas Cruz, described a volatile teenager whose strange behavior had caused others to end friendships with him, particularly after the fight that led to his expulsion.

Israel said Cruz was taken under arrest without incident. Authorities believe he acted alone.

“He was found off campus… I believe he was found in the city of Coral Springs,” he said.

Cruz was not a current student, but had been expelled according to authorities.

Dakota Mentcher, a junior, said he used to be friends with Cruz. But he cut off the friendship as Cruz’s behavior “started progressively getting a little more weird.” Cruz posted on Instagram about killing animals and threatened one of Mentcher’s friends, he said.

He remembered that Cruz had a pellet gun and did target practice in his backyard.

Pictures posted to social media depict the alleged gunman holding knives and a firearm, oftentimes with his face covered.

“We actually, a lot of kids joked around like that, saying that he was going to be the one to shoot up the school,” one student said. “It turns out that everyone predicted it. That’s crazy.”

Cruz “was that weird kid that you see … like a loner,” student Daniel Huerfano said.

U.S. Sen. Bill Nelson (D-Florida) said he had “no idea” where the suspect obtained the weapons.

Video from inside of the school showed screaming students hiding on the floor as shots were fired in the background.

Students leaving the building told a reporter for CBS4 Miami that the shooting took place inside a building known as ‘the freshman building.’

“We heard the pops when we were in the building, toward the back of the school close to the tennis court,” one student said. Others remained barricaded inside and began to make phone calls to their parents.

The parent of a student called CBS News and said that his daughter was hiding in a closet with her friends.

“She’s with a few of her friends,” Cesar Figueroa said.

He said his daughter and friends took cover as soon as the shooting started.

Figueroa told CBS News that he encountered a student outside the school who saw blood and bodies on the floor.

Jeiella Dodoo told CBS News she was able to escape the building.

“So we’re in fourth period, about to leave,” she said, “Then the fire alarm goes off, we thought it was a drill.”

She said they had already had a drill earlier that morning.

“We were waiting and then we heard gun shots,” she said.

Dodoo said they initially thought it was an active shooter drill. She and classmates took off running and sought safety at a local Walmart.

President Donald Trump took to Twitter to offer his condolences, and U.S. Attorney Ben Greenberg issued a statement as well.

“Our thoughts and prayers go out to the victims of this horrific act of violence. We have been in contact with our federal, state, and local law enforcement partners and stand ready to assist their efforts with all of the resources of this office.”

U.S. Sen. Marco Rubio (R-Florida) said, “today is the terrible day you pray never comes.”

The FBI responded to the scene to assist local law enforcement. The NYPD said they were monitoring the incident. Meanwhile, officials say Marjory Stoneman Douglas High School will remain closed the rest of the week.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)