Seventeen people have been killed, police have said, and a further 14 injured in a Florida high school shooting.
The attack began at Marjory Stoneman Douglas High School in Parkland, northwest of Fort Lauderdale, just before 3pm on Wednesday afternoon, local time.
Wearing a gas mask and carrying at least one AR-15 assault rifle, smoke grenades and several magazines of ammunition, the gunman deliberately set off the fire alarm so youngsters would leave their classrooms as he arrived at the scene, Florida Senator Bill Nelson told CNN.
Twelve of those who died were killed inside the building, two were shot dead outside, one in the street and two more died in hospital.
The school, which has around 3,000 students, was placed on a “code red” lockdown and television footage later showed dozens of people running and walking away from the school.
Broward County Sheriff Scott Israel later confirmed that the suspected gunman – named as 19-year-old Nikolas Cruz – was arrested off campus “without incident”.
Sheriff Israel said the suspect, a former student, was previously expelled for disciplinary reasons.
It is believed to be the second deadliest school shooting in US history, after Sandy Hook Elementary school in 2012.
Broward Country Schools Superintendent Robert Runcie said: “There are numerous fatalities. It’s a horrific situation.”
The sheriff’s office said 16 injured victims were taken to Broward Health Medical Center and Broward Health North hospital.
Two at Broward Health North died, three are in a critical condition, and three in a stable condition, doctors said. The condition of eight others is unknown.
Video has emerged that shows students cowering in a classroom as loud rapid shots are heard nearby.
McKenzie Hartley, 19, said her sister was inside the school at the time and she told Reuters: “She heard him shooting through the windows of classrooms and two students were shot.”
Kyle Yeoward, 16, told Reuters he was in the bathroom on the second floor of the building where most of the shooting took place, when he heard two shots.
“He let loose on the freshman building,” Kyle said.
Lissette Rozenblat, whose daughter goes to the school, told CNN that her daughter was safe but had heard the cries of a person who was shot.
“She was very nervous. She said that she could hear the person who was shot crying out for help.”
Former classmates told Reuters Cruz as a troubled former student who loved guns.
Chad Williams, 18, who is in Year 12 at Stoneman Douglas, remembered Cruz from when they attended middle school together.
He said Cruz would set off the fire alarm, day after day, and finally got expelled in the eighth grade.
More recently, Mr Williams saw Cruz carrying several publications about guns when they met each other at the high school.
Mr Williams said he believed Cruz was there to pick up a younger sibling.
“He was crazy about guns,” he said. “He was kind of an outcast. He didn’t have many friends. He would do anything crazy for a laugh, but he was trouble.”
Sky’s US Correspondent Cordelia Lynch said: “Just yesterday this city, Parkland, was named as the safest in Florida and now this terrible incident.
“Since 2013, there have been 300 school shootings.
“That gives you a sense of just how stark and tragically common these shootings can be.”
Florida congressman Ted Deutch echoed those thoughts, writing on Twitter: “Parkland is an idyllic community.
“Tragedies like this occur everywhere.
“School shootings occur everywhere.
“This is horrific.”
Florida Governor Rick Scott said on Twitter: “Just spoke with (US President Donald Trump) about shooting at Marjory Stoneman Douglas High School.
“My thoughts and prayers are with the students, their families and the entire community.”
Mr 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.”
(c) Sky News 2018: Seventeen dead in Florida high school shooting