By Faith Karimi and Emanuella Grinberg, CNN

(CNN) A former student armed with a rifle stalked the halls of a Florida school, breaking windows and shooting terrified students in a massacre that left 17 people dead Wednesday, authorities said.

The suspect, Nikolas Cruz, 19, had once been expelled from Marjory Stoneman Douglas High School over disciplinary problems, Broward County Public Schools Superintendent Robert Runcie said, without providing specifics.

Cruz allegedly turned on the fire alarm, sending many students running outside, where he fired shot after shot before following people inside the building, officials said.

There, he roamed the halls he knew so well, allegedly targeting those huddled in classrooms.

Former classmates said they were not surprised. He 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."

Cruz is believed to have concealed himself among the students as they fled the school, according to the Broward County Sheriff's Office. Investigators identified him from school security videos and arrested him later in nearby Coral Springs, according to police.

His first court appearance was set for Thursday morning.

The massacre

Just before gunshots erupted, the fire alarm blared.

At first, students were confused because there was 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 after they were hospitalized, Sheriff Scott Israel said.

The school

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 the classroom.

"There's nothing good to see in there," Baltzer said the officer told them.

All she saw was broken glass.

The victims

Authorities have not released the identities of the victims, but they include 17 adults and children.

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.

The suspect

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 adopted mother -- her cousin Lynda Cruz -- died in November of pneumonia while his father passed on years ago, according to Blaine.

"I haven't seen Lynda, we would talk on the phone occasionally," she said.

School closed

The school is closed for the rest of the week, Broward County Public Schools Superintendent Robert Runcie said. The district will offer grief counseling to students and their families.

"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," he said.

The shooting, which is among the 10 deadliest mass shootings in modern US history, 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.

CNN's Sara Ganim, Eric Levenson, Chris Boyette and Shimon Prokupecz contributed to this report.