The shooting at Marjory Stoneman Douglas High School in Parkland, a placid, middle-class community about 45 miles (72 km) north of Miami, ranked as the second-deadliest act of gun violence ever on a U.S. public school campus.

The attack began shortly before the school’s dismissal.

Television footage showed images, increasingly familiar in America, of bewildered students streaming out of the building with hands raised in the air, weaving their way between heavily armed, helmeted police officers, as a fire truck and other emergency vehicles idled nearby.

Florida’s two U.S. senators, briefed by federal law enforcement officials, said the assailant wore a gas mask as he stalked into the school carrying a rifle, ammunition cartridges and smoke grenades, then pulled a fire alarm, prompting students and staff to pour from their classrooms into hallways.

“There the carnage began,” Senator Bill Nelson told CNN. Senator Marco Rubio gave a similar account on Twitter.

A chilling cell phone video clip broadcast by CBS News showed a brief scene of what the network said was the shooting in progress from inside a classroom, where several students were seen huddled or lying on the floor surrounded by mostly empty desks. A rapid series of loud gunshots are heard amid hysterical screaming and someone yelling, “Oh my God.”

The gunman was arrested later outside, 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.

He was identified as Nikolas Cruz, who previously attended the high school and was expelled for unspecified disciplinary reasons, Broward County Sheriff Scott Israel said at a news briefing hours later.

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 his “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 behavior at school, including a penchant for pulling false fire alarms, and was “crazy about guns.”

The gunman surrendered to police without a struggle, Israel said. He was armed with an AR-15-style rifle and had multiple magazines of ammunition.

A man placed in handcuffs is led by police near Marjory Stoneman Douglas High School following a shooting incident in Parkland, Florida, February 14, 2018 in a still image from video. WSVN.com via REUTERS
“It’s catastrophic,” Israel said. “There really are no words.” Broward County Schools Superintendent Robert Runcie called it “a horrific situation,”

Twelve of the dead were killed inside the school, two others just outside, one more on the street and two other victims died from their wounds at a hospital, Israel said. He said the victims comprised a mixture of students and adults.

The sheriff said at a briefing after dark that investigators had so far positively identified just 12 of the dead, including a football coach he did not name.

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.

The Valentine’s Day bloodshed in the racially diverse Miami suburb of gated communities with palm- and shrub-lined streets was the latest outbreak of gun violence that has become a regular occurrence at schools and college campuses across the United States over the past several years.

It was the 18th shooting in a U.S. school so far this year, according to gun control group Everytown for Gun Safety. That tally includes suicides and non-injury incidents, as well as a January shooting in which a 15-year-old gunman killed two fellow students at a Kentucky, high school.

Wednesday’s violence marks the second-greatest loss of life from a shooting at a U.S. public school, after the 2012 massacre of 20 first-graders and six educators at Sandy Hook Elementary in Newtown, Connecticut, by a gunman who also killed his mother and himself.

It also is the deadliest ever at an American high school, surpassing the 1999 rampage at Columbine High School in Littleton, Colorado, where two teenagers killed 12 students and a teacher before taking their own lives.

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 classrooms.

One survivor, Kyle Yeoward, 16, a junior, told Reuters he and about 15 fellow students and a teacher hid in a closet for nearly two hours before police arrived. Yeoward said most of the shooting occurred in the building for the school’s freshman class.

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 had heard the cries of a person who was shot.

The school had recently held a meeting to discuss what to do in such an attack, Ryan Gott, a 15-year-old freshman, told CNN.

“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.”

The school, named for a famed Florida environmental activist and author, was to remain closed for the rest of the week, education officials said.

More about: #Florida