Florida school massacre: 17 shot dead, expelled student taken into custody

WARNING - DISTRESSING CONTENT: Seventeen people have been killed and more injured after a gunman opened fire at a US high school.

Police swarmed the Marjory Stoneman Douglas High School in Parkland, Florida on Wednesday afternoon (local time) after multiple emergency calls were made from panicked students.

One man, matching a description given by children and believed to be a former student, has since been taken into custody. A federal official has identified the suspect as Nicolas Cruz.

"There are numerous fatalities. It's a horrific situation," Broward Country Schools Superintendent Robert Runcie said.

He said the alleged perpetrator used to attend the school and is believed to be 16 years old.

Students hid in classrooms, with one child sharing images from inside a classroom, until they were rescued by police in tactical gear, friends and family members said.

Twelve of the dead were killed inside the school building, two others just outside, one more on the street and two other victims died of their injuries at a hospital, Broward County Sheriff Scott Israel said

He said the victims comprised a mixture of students and adults.

"There was a time when he did attend the school," Israel said.

"I don't know why he left, I don't know when he left."

Children were marched from the school in a single file row with their hands raised above their heads. Source: AAP

Paramedics rush an injured person from the high school. Source: AAP

Some students exited the building in single-file rows with hands raised overhead to show they carried no weapons.

"What happened was that the fire bell rang and we all thought it was a drill because we had already had a fire drill," an unidentified 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.

Others held onto other students as they made their way out past helmeted police in camouflage with weapons drawn.

Students said they initially thought it was a fire drill. Source: AAP

The school has been placed on a "code red" lockdown. Source: 7 News

Police officers in helmets, bulletproof vests and armed with automatic weapons could be seen stationed at several points around the school.

TV footage showed at least one person being wheeled to an ambulance on a gurney while emergency workers appeared to be helping others on the sidewalk.

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

President Donald Trump offered his condolences to those impacted by the shooting.