The shooting, which is among the 10 deadliest mass shootings in modern US history, revived debate over gun control while American politicians offered their condolences.

“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,” US President Donald Trump said.

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 a fire drill earlier in the day, leading some to think it was a false alarm.

While some left the building, others sought cover in classrooms as the school went into lockdown and the gunman went on his rampage.

Some students, fearing their lives would soon be over, texted goodbyes to loved ones; others used their phones to share startling footage of the carnage on social media.

One teacher said she was on her way out of the building after the fire alarm when another staff member told her the situation was code red — an active shooter. Melissa Falkowski returned to a classroom and hid in a closet with 19 students from her newspaper class for nearly 40 minutes.

Falkowski credited the fire drill and recent active shooter 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 to still have so many casualties, at least for me, it’s very emotional. Because I feel today that our government, our country has failed us and failed our kids and didn’t keep us safe.”