The suspect, who police have now taken into custody, is a 19-year-old former student, according to the Miami Herald newspaper, external.

"We were told last year that he wasn’t allowed on campus with a backpack on him," said maths teacher Jim Gard.

"There were problems with him last year threatening students, and I guess he was asked to leave campus," he added.

Police have yet to formally identify the suspect, but images have shown them taking a young man wearing a burgundy shirt into custody.

He was taken under arrest without incident, according to Sheriff Israel.

He adds that the shooter is "approximately 18 years old".

Broward County Sheriff Scott Israel says the suspect was found and arrested "off-campus".

"From what I understand, there was a time where he did attend the school. I don't know why he left, I don't know when he left," Mr Israel said.

Police have yet to identify a suspect, but a man placed in handcuffs was seen being loaded onto stretcher and placed into a paramedic vehicle.

Image source, Reuters

Image source, Reuters

Image source, Reuters

Parents were seen forming prayer circles outside the school amid the chaos, according to a Daily Beast reporter at the scene.

Connecticut Congresswoman Elizabeth Esty is reminding Americans that these sort of incidents only ever seem to happen in the US.

According to research, external conducted by Everytown for Gun Safety, this is at least the 18th shooting in the US this year that occurred on or around school premises.

Since 2013, there have been 291 reported school shootings in America, which averages out to about one per week.

On 24 January 2017, two people were killed and 17 injured at a high school in Kentucky.

Image source, Reuters

The Broward County Sheriff's Office confirmed at 4:22pm EST so far they have at least 14 victims transported to hospital.

Broward County Public Schools Superintendent Robert Runcie said: "There are numerous fatalities. It's a horrific situation.

"It's just a horrible day for us."

He adds: "We have no evidence at this point there was more than one shooter."

"It is a day that you pray every day that you never have to see," he says.

Of the suspect: "We didn’t have any phone calls or threats that we know of that were made."

He added: "We cannot live in a world built on fear, we have do what we can to provide the greatest safety measures for our kids.

"Mental health issues in this country are growing and it’s certainly something that needs to be addressed in our school system."

CBS posted extremely distressing footage in which screams and gunfire were heard from inside the school as students took shelter in a classroom.

The network later took down the clip, deciding editorially that it was too graphic.

The sprawling campus is located about 45 miles (70km) north of downtown Miami.

According to the National Center for Education Statistics, the school population in the 2016-17 academic year was just over 3,100 students.

A man who identified himself as Caesar Figueroa told CBS News his daughter was still inside the school.

"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 there."

The US First Lady was at a Valentine's Day event with children at a school in Maryland when news of the shooting broke.

A spokeswoman for Mrs Trump said that she had been made aware of the shooting, but made no comments about it to the children.

Florida Senator Bill Nelson told Fox News that "there are many deaths" following the shooting at Marjory Stoneman Douglass High School.

CBS Miami, Fox affiliate WZDX and a Florida state representative have reported the shooter is in custody.

Parkland, where the shooting took place, was ranked the third safest city in Florida, external, according to the National Council for home safety and security.

The city, which boasts 30,000 people, is located between West Palm Beach and Fort Lauderdale, near Coral Springs.

"We thought it was a drill," a student evacuated from the high school tells CBS Miami.

"We already had one earlier this morning... and then we heard gunshots; some students thought it was not that serious."

The father of one student tells the station that his daughter heard windows shattering.

"She said she screamed and ran into the closet with her friends," adds the distraught father.

Another man tells CBS that a suspect was "wearing a black vest with a red shirt".

Any information about the attacker has yet to be confirmed by police.

"Supposedly he pulled the fire alarm and planned it," the man adds.

The White House has reportedly cancelled the daily press conference in the light of the unfolding tragedy in Florida.