There are now reports of multiple casualties including fatalities amid a shooting at a high school in the United States.
Police in Broward County, Florida, have been responding to the attack at Marjory Stoneman Douglas High School in Parkland.
A spokeswoman for the sheriff's department said the school had been placed on a "code red" lockdown.
The Broward County sheriff's spokesman has said that the suspect is in custody, following earlier reports that the shooter was at large.
"There are numerous fatalities. It's a horrific situation," Broward County Schools Superintendent Robert Runcie told reporters.
Earlier, Florida Senator Bill Nelson told NBC News there were a "number of fatalities" in the school shooting, citing a school official.
The Broward Sheriff has tweeted to say that "so far we have at least 14 victims" who were being taken to local hospitals. He did not give details of their condition.
Parkland lies to the north of Miama and some 25 kilometres west of Fort Lauderdale.
Broward Sheriff Scott Israel has appeared before TV cameras to say the suspect is believed to be a former student at the school, aged about 18. "It's catastrophic. There really are no words," he later tweeted.
Dozens of students have been seen fleeing a building surrounded by police. Live television showed them running and walking away from the school. As the school went on lockdown, the school began sending students home as police cleared them from "one building at a time". The local police department tweeted to say that teachers and students should remain barricaded inside until reached by police.
Stories began to emerge from those who managed to get clear of the premises. Friends and family members said students hid in classrooms until they were rescued by police in tactical gear.
Someone identified as the sister of a student at the school described the scene in a text to Reuters. "She heard him shooting through the windows of classrooms and two students were shot," said 19-year-old McKenzie Hartley.
The mother of another student said her daughter could hear a person who was shot, crying for help.
The FBI has been called in and is understood to be helping local officers.
Florida Governor Rick Scott said he had spoken to President Trump, who has tweeted his condolences.
Multiple shootings... in 2018
CNN has also reported that there have been 18 incidents of school shootings in the US this year.
A 15-year-old gunman killed two students at a high school in Benton, Kentucky, in January, Reuters reports.
Students at the Marjory Stoneman Douglas High School had recently held a meeting to discuss what to do in case of such an attack.