At least 17 people were killed Wednesday in a high school shooting in Parkland, Florida, the Broward County Sheriff's office said.
The suspect, 19-year-old former student Nikolas Cruz, is in custody. Cruz was expelled for unspecified disciplinary reasons, and police are investigating his digital footprint, Sheriff Scott Israel said. So far, what they've found is "very, very disturbing," he said.
"It's a horrific, horrific day," Israel said. "Just pray for this city. Pray for this school, the parents, the folks that lost their lives."
Law enforcement responded to reports of a shooting at Marjory Stoneman Douglas High School shortly before 3 p.m. and encountered a chaotic scene.
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. But the school already had performed a fire drill earlier in the day, leading some to think it was a false alarm.
As some left the building, others sought cover in classrooms as the school went into lockdown. Students used their phones to text loved ones and share videos and pictures on social media. One teacher, Melissa Falkowski, said she hid in a closet with 19 students from her newspaper class.
Falkowski credited the fire drill and other forms of training with saving lives. And yet, it didn't save enough.
"We could not have been more prepared for this situation, which is what makes it so frustrating," Falkowski told CNN's Anderson Cooper.
"We did everything that we were supposed to do. Broward County Schools has prepared us for this situation and still to have so many casualties ... for me, it's very emotional because I feel today like our government, our country has failed us and failed our kids and didn't keep us safe."
Law enforcement sources said a .223 caliber, AR-15 style firearm was used in the shooting.
Freshman Kayden Hanafi said he heard two gunshots and saw people running out of another building on campus. As he and his classmates went into lockdown in a classroom, many thought the noise might have been firecrackers.
"It's really a blessing to still be alive," he said.
Nicole Baltzer, 18, said she was in trigonometry class about 10 minutes before the end of the school day when the fire alarm went off. As students evacuated, she heard six gunshots and everyone started running back inside the school, Baltzer said.
"I heard so many gunshots, at least like six. They were very close," Baltzer said.
A police officer told her to close her eyes as she walked past a classroom with broken glass, telling her "there's nothing good to see in there," she said.
A freshman named Aidan posted a photo on social media from inside his math class while on lockdown.
"We have been liberated. God bless, America," Aidan tweeted after being evacuated from the building. "Love each other. You may never know when it may be the last day you meet someone."
The shooting sent students and staff streaming out of the school. Some walked calmly with their hands up while others ran. According to a law enforcement source, the suspect initially mixed in with the crowd of students to try to get away, but it didn't work.
Immediately after the shooting, aerial footage from CNN affiliate WSVN showed people lying on the ground outside the school, being treated for injuries and moved to ambulances. Seventeen people, including the suspect, were sent to area hospitals, said Dr. Evan Boyar of Broward Health.
As news of the shooting spread, desperate parents gathered near the property, searching for their children.
Those who died included students and adults, the sheriff said. Twelve were killed inside the building and two died outside, he said. One died in the street and two died at the hospital, Israel said.
Parkland, with a population of 31,000 in 2016, was named Florida's safest city last year, according to one analysis. The south Florida city had seven reported violent crimes and 186 property crimes the previous year, the analysis said.
By Emanuella Grinberg, Eric Levenson and Shimon Prokupecz, CNN