A former, 19-year-old, student at Florida high school returned to the premises where he had once been expelled for disciplinary problems and opened fire with an assault-style rifle on Wednesday, killing 17 people and injuring more than a dozen others before he was arrested, authorities said.
The violence erupted shortly before dismissal at Marjory Stoneman Douglas High School in Parkland, a placid, middle-class community about 72km north of Miami. Television coverage showed images, increasingly familiar in America, of bewildered students streaming out of the building with their hands in the air, as dozens of police and emergency services personnel swarmed the area.
Florida's two U.S. senators, briefed by federal law enforcement officials said the assailant wore a gas mask as he slaked into the school carrying a rifle, ammunition and smoke grenades, where he then pulled a fire alarm, prompting students and staff to pour from the classrooms into hallways.
"There the carnage began," Senator Bill Nelson told CNN. Senator Marco Rubio gave a similar account on Twitter.
CBS News broadcasted a chilling cellphone video that showed a brief scene of what the network said was the shooting in progress from inside a classroom, where several students were seen lying on the floor surrounded by empty desks. A rapid series of loud gunshots are heard followed by hysterical screaming.
The gunman was arrested later, some distance from the school in an adjacent community. CNN, citing law enforcement sources, said the gunman tried to blend in with students who were fleeing the school but was spotted and taken into custody.
The shooter was identified as Nikolas Cruz, who previously attended the high. As a high school freshman, Cruz was part of the U.S. military-sponsored Junior Reserve Officers' Training Corp program at the school, according to Jillian Davis, 19, a recent graduate and former fellow JROTC member at Stoneman Douglas High.
In an interview with Reuters, Davis recalled "strange talking sometimes about knives and guns," adding, "no one ever took him seriously."
Chad Williams, 18, a senior at Stoneman Douglas, described Cruz as "kind of an outcast" who was known for unruly behaviour at school, including a penchant for pulling false fire alarms, and was "crazy about guns."
The gunman surrendered to police without struggle and was armed with an AR-15-style rifle and had multiple magazines of ammunition.
Twelve of the dead were killed on school premises, two others just outside, one more on the street and two other victims died of their injuries at a nearby hospital. The victims comprised a mixture of students and adults. Authorities at two nearby hospitals said they were treating 13 survivors for bullet wounds and other injuries, five of whom were listed in critical condition.
This was the 18th shooting in U.S. schools so far, this year alone, according to gun control group Everytown for Gun Safety. This includes suicides and incidents when no one was injured, as well as the January shooting in which 15-year-old gunman killed two fellow students at a Benton, Kentucky, high school.
Staff and students told local media that a fire alarm went off around the time the shooting started, sparking chaos as some 3,300 students at the school first headed into hallways before teachers herded them back into the classrooms, to seek shelter in closets.
Anguished parents checked on their children.
"It is just absolutely horrifying. I can't believe this is happening," Lissette Rozenblat, whose daughter goes to the school, told CNN. Her daughter called her to say she was safe but the student also told her mother she heard the cries of a person who was shot.
"My prayers and condolences to the families of the victims of the terrible Florida shooting," U.S. President Donald Trump said on Twitter. "No child, teacher or anyone else should ever feel unsafe in an American school."