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Frantic parents rushed to the scene and ambulances converged in front of the Marjory Stoneman Douglas High School. Officials say the shooting began around 2:20 p.m. EST and the suspect was taken into custody a little less than two hours later.
Law enforcement sources say at least 17 people - both students and adults - died in the shooting. Numerous other people were injured.
The suspect was identified as Nikolas Cruz, 19, a former student who had been expelled from the school. He was armed with an AR-15 and numerous magazines, officials said.
VIDEO: Student at school where shooting happened: 'Everyone predicted this'
Broward County Sheriff Scott Israel said the suspect was arrested without incident after he was located off the school grounds in a nearby community about two hours after the shooting.
A family member of Nikolas Cruz tells ABC News that Nikolas was adopted by Roger and Lynda Cruz - both of whom are now deceased - along with a younger brother. His adopted mother passed away last year.
Seventeen-year-old junior Matthew Walker spoke to ABC News, saying the suspect was known to show off knives and guns.
"He was going class to class just shooting at random kids," he said. "Everything he posts (on social media) is about weapons. It's sick."
Robert Runcie, superintendent of Broward County Public Schools, confirmed "numerous fatalities."
VIDEO: GRAPHIC: Harrowing video shows students cowering as gunshots ring out
Florida school shooting: Harrowing video shows students cowering as gunshots ring out
"It is a horrific situation." He added, "It is a horrible day for us."
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Aerial footage showed police in olive fatigues, with weapons drawn, entering the high school, then dozens of children frantically running and walking quickly out. A police officer waved the students on, urging them to quickly evacuate the school.
Some students exited the building in single-file rows with hands raised overhead to show they carried no weapons. Others held onto other students as they made their way out past helmeted police in camouflage with weapons drawn.
Emergency medical personnel pulled stretchers from the backs of ambulances as police cars surrounded the parking lot.
VIDEO: School shooting witness: I heard too many gunshots to count
School shooting witness: I heard too many gunshots to count
Of the 17 casualties, officials say 12 died inside the school, two outside, one on the street and two at the hospital. The shooting began outside the school and then continued on the inside.
One eyewitness, student Michael Katz, said he saw at least two bodies as he evacuated the school. Katz, who was in the first wave of students to leave, estimated that hundreds -- if not a couple thousand -- high schoolers were still in the building well into the shooting.
Approximately 3,000 students attend the high school and another 1,500 go to the adjacent middle school.
Marjory Stoneman Douglas High School released a statement Wednesday afternoon that reads: "Close to dismissal today at Marjory Stoneman Douglas High School, staff and students heard what sounded like gunfire. The school went on an immediate code red lock down. We are receiving reports of possible multiple injuries. The school remains on lock down. Law enforcement and the District's Special Investigative Unit are currently on site. The District will provide updates as more information becomes available."
Katz reported hearing gunshots for "three-and-a-half to four minutes." Police, he said, arrived about 20 minutes after the shooting began.
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Katz said he did not see the shooter and did not get the impression that there were multiple people carrying out the attack.
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Len Murray's 17-year-old son, a junior at the South Florida high school where shooting was reported, sent his parents a chilling text: "Mom and Dad, there have been shots fired on campus at school. There are police sirens outside. I'm in the auditorium and the doors are locked."
Those words came at 2:30 p.m. Wednesday. A few minutes later, he texted again: "I'm fine."
President Donald Trump responded quickly on Twitter to the shooting. He wrote, "My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school."
President Donald Trump responded quickly on Twitter to the shooting. He wrote, "My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school."
Click here for full coverage on the tragedy at Marjory Stoneman Douglas High School in Parkland and click here for a look at comprehensive coverage on school shootings here in the Bay Area and across the country.
PHOTOS: Shooting at Florida high school
ABC7 and The Associated Press contributed to this report