A gunman unleashed horror at his former high school in South Florida on Wednesday, authorities said, slaughtering at least 17 unsuspecting students and adults and leaving the country asking why another school massacre has happened yet again.
The suspect, Nikolas Cruz, 19, gunned down victims with a rifle at Marjory Stoneman Douglas High School in Parkland shortly before 3 p.m. after he allegedly activated a fire alarm, sending people outside unaware of what was to come, officials said.
He fired shot after shot outside and then followed others running back inside the building, where people who'd heard the shots were taking shelter, officials said.
There, he roamed the halls he knew so well, allegedly targeting those huddled in classrooms and then blending in with the students and staff evacuating the school. He was arrested in the neighboring community of Coral Springs later in the day, the Broward County Sheriff's Office said.
Cruz had once been expelled from the high school over disciplinary problems, Broward County Public Schools Superintendent Robert Runcie said, without providing specifics.
The young man now stands accused of committing one of the 10 deadliest mass shootings in modern US history. He has been charged with 17 counts of premeditated murder, the sheriff's office said.
"This has been a day where we've seen the worst of humanity. Tomorrow is gonna bring out the best in humanity as we come together to move forward from this unspeakable tragedy," Runcie said.
Former classmates said they were not surprised at the identity of the suspected shooter. Cruz loved showing off guns, student Eddie Bonilla told CNN affiliate WFOR.
"We actually, a lot of kids threw jokes around Iike that, saying that he's the one to shoot up the school, but it turns out everyone predicted it. It's crazy," Bonilla recalled.
Alex Azar also said he remembered Cruz.
"He always seemed like the unstable type, the type who would do this sort of thing," Azar told WFOR. "He was always in trouble. ... He had that look to him, kind of sinister."
Investigators identified Cruz from school security videos before arresting him in Coral Springs, according to police.
His first court appearance was expected Thursday afternoon.
Just before gunshots erupted, the fire alarm blared.
At first, students were confused because there had been a fire drill earlier that day. Investigators believe the suspect pulled the fire alarm to draw people out of classrooms and increase the number of casualties, a law enforcement source told CNN
When some students and teachers rushed out of the building, the massacre began. Others scurried back into classrooms as the school went into code red, signaling an active shooter.
Some texted goodbyes to loved ones, fearful they wouldn't make it out alive. Others posted social media images of chairs overturned in classrooms and floors stained with blood.
In one cell phone video, a student cried, "Oh my God! Oh my God!" as gunshots popped in rapid succession in the background.
By the end of the rampage, 12 people were killed inside the building, two outside and one on the street. An additional two people shot at the school died at the hospital, Sheriff Scott Israel said.
An undetermined number of others were injured, officials said.
A little before 3 p.m. ET, the Broward County Sheriff's Office responded to reports of a shooting at the school.
"We are receiving reports of possible multiple injuries. Law enforcement is on site," it said on Twitter. Law enforcement officers rushed to the school, where they found a grim scene.
Beyond the bodies and blood lay piles of backpacks and dropped cell phones, some still ringing as desperate parents tried to reach their children, a source told CNN.
Nicole Baltzer, 18, was in trigonometry class when the shooting started. When a SWAT team arrived to rescue them, she said, an officer requested they close their eyes as they passed by another classroom.
"There's nothing good to see in there," Baltzer said the officer told them.
All she saw was broken glass.
Authorities have not released the identities of the victims, but they include 17 adults and children.
Football coach Aaron Feis was among the dead, according to the school's football program and its spokeswoman, Denise Lehtio.
He threw himself in front of students as bullets hailed down Wednesday at his alma mater. It would become perhaps the final act of this assistant coach and security guard, who suffered a gunshot wound and later died after he was rushed into surgery,
"He died the same way he lived -- he put himself second," Lehtio said. "He was a very kind soul, a very nice man. He died a hero."
Loved ones posted photos of missing students on social media, seeking their whereabouts.
"This is Jamie Guttenberg a student a Stoneman Douglas High. If you see her please have her call her parents ASAP. Praying for all of the families and kids," Michelle Wiseman Kredi posted on Facebook.
Police are investigating Cruz's digital footprint, and what they've found so far is "very, very disturbing," Israel said.
The suspect used a .223 caliber, AR-15 style firearm in the shooting, which he bought in the past year after passing a background check, a source told CNN's Evan Perez.
Kathie Blaine, a cousin of Cruz's mother, said he was adopted and she had never met him.
His adoptive mother -- her cousin Lynda Cruz -- died in November of pneumonia while his father passed away years ago, according to Blaine.
"I haven't seen Lynda, we would talk on the phone occasionally," she said.
The school is closed for the rest of the week, Runcie, the superintendent, said. The district will offer grief counseling to students and their families.
The shooting has revived debate over gun control.
Parkland, with a population of 31,000 in 2016, was named Florida's safest city last year. It had seven reported violent crimes and 186 property crimes the previous year, an analysis said.