WARNING, DISTRESSING CONTENT: Terrified students trapped in a Florida high school have shared photos from inside their classroom as a gunman made his way through the grounds killing at least 17 people and injuring several more.
Shortly before students were to be dismissed for the day, the sound of gunfire ripped through the air, sending hundreds of students running for their lives.
Live television showed dozens of students, weaving their way between police officers with heavy weapons and helmets, and large numbers of emergency vehicles including police cars, ambulances and fire trucks.
The suspected gunman was identified as Nikolaus Cruz, 19, who previously attended the school and was expelled for unspecified disciplinary reasons.
During the shooting one student gave an insight into the fear he and his classmates were experiencing, sharing images of frightened students huddled up against the back wall of their classroom.
"All of a sudden I heard, 'Boom, boom, boom!' and I heard screaming everywhere.
"We fell underneath my teacher's desk. The shooter was literally feet outside of my classroom," student Michael Katz told ABC News.
A person who identified herself as the sister of a student at the school said her brother heard the gunman "shooting through the windows of classrooms and two students were shot".
"What happened was that the fire bell rang, and we all thought it was a drill because we had already had a fire drill," another student told WSVN 7 News.
"We thought it's another drill so we didn't take it seriously and then we heard pops from the other side of the school," the student said.
Panicked parents could only watch on from a distance and hope for the best.
"My daughter, as of right now, she's still trapped in a closet. She's afraid to speak," Caesar Figueroa told CBS News.
"I told her, 'don't call me, because I don't want no one to hear your voice.' So, she's still trapped in a closet in there."
Since the 2012 massacre at the Sandy Hook Elementary School in Newtown, Connecticut, where 20 children and six adults were shot dead, warning procedures and emergency drills have multiplied at US schools.
The goal is to teach school children how to react to a shooter who opens fire at random.