Emergency responders at a school in Parkland, Florida. WTVJ

PARKLAND, FL – A gunman opened fire at a Florida high school on Wednesday afternoon, and there are “a number of fatalities,” according to Sen. Bill Nelson, D-Fla.

Authorities responded to an active shooter at Marjory Stoneman Douglas High School in Parkland around 2:30 p.m. and said that the gunman was not in custody.

More than an hour later, just after 4 p.m., the Broward County Sheriff’s Office announced on Twitter that a suspect had been apprehended, adding that the scene was still active.

Citing Broward County officials, Nelson told NBC News that there were multiple fatalities.

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“This is a really bad day,” the senator said.

Earlier, the sheriff’s office tweeted that “there are reports of victims,” but did not say how many people were injured. Broward County Public Schools said on Twitter that the sound of gunfire broke out shortly before school was dismissed for the day, and that it received reports of “possible multiple injuries.”

The high school went on immediate lockdown, Parkland Mayor Christine Hunschofsky told NBC Miami. She did not have details on casualties or who the shooter was.

“It’s sad. It’s sad that these tragedies happen in our country,” she said. “Many of the students have been in touch with their parents. We have many, many parents out here.”

Parents told the station that students were hiding inside closets at the school, and that teachers had told them not to leave the building.

Joel Leffler, who has a son and a daughter at the school, said both of his children were safe but in shock.

“My son called me as it was unfolding, running. He had to jump a fence,” Leffler told NBC News. “My son heard around eight gun shots as he was running out.”

When he reached his daughter, she was whispering, he said.

“My daughter, who was there in the freshman hall where the shooting took place — she’s in shock right now and she’s being taken out by SWAT,” Leffler said. “She saw multiple dead bodies.”

FBI agents from the Miami division were en route, officials said, as were agents from the Miami Field Division of the Bureau of Alcohol, Tobacco, Firearms and Explosives.

President Trump has been briefed on the incident and the White House was monitoring the situation, White House press secretary Sarah Huckabee Sanders said.

Sen. Marco Rubio, R-Fla., also tweeted that he was “monitoring the horrible unfolding situation in Broward County, Florida with reports of a shooting at a local high school.”

Florida Gov. Rick Scott was traveling to Broward County to be briefed by emergency management officials and law enforcement, and has been in touch with Trump and the Department of Homeland Security, his office said.

Parkland, in north Broward County, is about 30 miles northwest of Fort Lauderdale.