LIVE COVERAGE: At least 17 dead after mass shooting at Florida high school
Students leaving Marjory Stoneman Douglas High School (Screenshot)

At least one person is dead and 20 injured after a shooter opened fire at a high school in Florida on Wednesday afternoon.

One person has died, a local fire chief told the Miami Herald. Florida Senator Bill Nelson (D) told Fox News there had been "many deaths."

Authorities not confirmed the number of injuries or fatalities.

WTVJ reported that the shooter intentionally pulled the fire alarm at Marjory Stoneman Douglas High School in Parkland, then opened fire on students.

"We thought it was a drill," one student told CBS News. "We already had one earlier this morning... and then we heard gunshots, some students thought it was not that serious."

A SWAT team has responded to the incident. "Remain barricaded inside until police reach you," police told students and faculty.

"My daughter, as of right now, she's still trapped in a closet. She's afraid to speak," a parent told CBS News.

Margate Fire Rescue had deemed the shooting a mass casualty incident.

Police said a suspect is in custody. The suspect, who has not been named, is a male student and his social media activity indicates he is a member of "a number of gun groups," according to Fox News.

"He’s been a troubled kid, and he’s always had a certain amount of issues going on. He shot guns because he felt it gave him, I guess, an exhilarating feeling," a student who knew the suspect told WSVN.

"I stayed clear of him most of the time. My time in alternate school, I did not want to be with him at all because I didn’t want to cause any conflict with him because of the impression he gave off."

The suspect has been taken to a hospital.

President Donald Trump tweeted: "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."

Watch live video, courtesy of CBS Miami below:

UPDATE -- 5:02 PM

The Broward County sheriff said the shooter is believed to be a former student at Marjory Stoneman Douglas High School.

UPDATE -- 5:56 PM

ABC News reports at least 15 people are dead after a mass shooting at a Florida high school on Wednesday.

UPDATE -- 6:27 PM

As a press conference, Broward County Sheriff Scott Israel said 17 people have been confirmed dead after a mass shooting at Marjory Stoneman Douglas High School.