The former student accused of shooting dead 17 people at a Florida High School allegedly set off the fire alarm to drive students into hallways before he started his rampage.

Police say Nikolaus Cruz, 19, had once been expelled for disciplinary reasons from Stoneman Douglas High School in Parkland, a community about 72 kilometres north of Miami.

Cruz allegedly opened fire with an assault-style rifle Thursday morning (AEDT) killing 17 people and injuring more than a dozen others before being arrested nearby without incident.

US Senator Bill Nelson of Florida told CNN the gunman wore a gas mask, was carrying smoke grenades and set off a fire alarm, prompting a mass evacuation.

“He set off the fire alarm so the kids would come pouring out of the classrooms and into the hall, and there the carnage began,” Senator Nelson said.

Twelve of the dead were killed inside the school, two others just outside, one more on the street and two other victims died of their injuries at a hospital.

As a high school freshman, Cruz was part of the US military-sponsored Junior Reserve Officers’ Training Corp program at the school, according to Jillian Davis, 19, a recent graduate and former fellow Corp member.

She recalled his “strange talking sometimes about knives and guns,” adding, “no one ever took him seriously.”

Students exit the Florida high school. Photo: AAP

Chad Williams, 18, a senior at Stoneman Douglas, described Cruz as “kind of an outcast” who was known for unruly behaviour at school and was “crazy about guns.”

Broward County Sheriff Scott Israel said 12 of the 17 people killed had been identified, but the bodies will remain at the scene until viewed by the medical examiner.

Mr Israel told media that the lone shooter was a 19-year-old former student Nicolas Cruz, who was armed with an AR-15-style rifle and multiple magazines of ammunition.

“He got expelled for disciplinary reasons, I don’t know the specifics,” Mr Israel said.

We are beginning to dissect his websites and things on social media that he was on and some of the things that have come to light are very, very disturbing.

“I don’t know anything about the firearm. He had countless magazines, multiple magazines and at this point we believe he had one AR rifle, I don’t know if he had a second one.”

Mr Israel added: “It’s catastrophic. There really are no words.”

Police arrest a man near the Florida school. Photo: Twitter

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.

It was the 18th shooting in a US school so far this year, according to gun control group Everytown for Gun Safety.

Freshman building attacked first

Kyle Yeoward, a 16-year-old junior, said he was in the bathroom on the second floor of a building when he heard two shots.

“He let loose on the freshman building,” Yeoward said.

CBS News posted a brief clip of cell phone video footage the network said was taken from inside a classroom, showing what appeared to be several students. A rapid series of loud gunshots are heard amid hysterical screaming and someone yelling, “Oh my God.”

The shooting has again reignited the gun debate in the US, although US President Donald Trump offered did not address those issues when he took to Twitter soon after the shooting.

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.

— Donald J. Trump (@realDonaldTrump) February 14, 2018

“My prayers and condolences to the families of the victims of the terrible Florida shooting,” he wrote.

“No child, teacher or anyone else should ever feel unsafe in an American school.”

Evan Boyar, a senior medical officer representing the local hospital network where many of the injured were taken said the suspect was also treated for unspecified injuries and released to police custody.

“Every patient that comes in get treated as a patient,” said Dr Boyer.

Shaken parents outside the high school. Photo: AAP

Live television during the shooting showed dozens of students running and walking away from the school, weaving their way between large numbers of emergency vehicles including police cars, ambulances and fire trucks.

Students led away from the scene. Photo: Getty

“It’s sad. It’s sad that these tragedies happen in our country,” Parkland Mayor Christine Hunschofsky said.

Stoneman Douglas High School English teacher Melissa Falkowski said she hid 19 students in a storage cupboard with her during the shooting.

She told CNN she and her fellow teachers recently undertook emergency training.

“If we didn’t have that training it could have been a lot worse,” she said.

As the students were evacuated from the school, disturbing video began to emerge of the situation inside the classrooms during the rampage.

JUST IN (warning, disturbing video): Cell phone video inside the school as shots were going off at Marjory Stoneman Douglas High https://t.co/wHWo6XjccX pic.twitter.com/3ovf5LhzBs

— CBS Evening News (@CBSEveningNews) February 14, 2018

More than 3100 students attend the Stoneman Douglas High School, which serves a community of just over 31,000 people that had only seven reported violent crimes and 186 property crimes in 2017.

-With AAP, ABC, agencies