At least 17 people are dead after a 19-year-old man opened fire at a high school campus in Parkland, Florida, police have said.
The suspect has been named as Nikolas Cruz, a former student at the school who had been expelled.
As the attack unfolded students were forced to hide as police swooped in on the building.
It is the deadliest school shooting since 26 people were killed at Connecticut school Sandy Hook in 2012.
It is the sixth school shooting incident in 2018 that has either wounded or killed students.
Broward County Sheriff Scott Israel told reporters Mr. Cruz killed three people outside the school, before entering the building and killing another 12.
Two people later died after being taken to the hospital.
"It's catastrophic. There really are no words," Sheriff Israel tweeted later.
Three people remain in a critical condition and three others are in stable condition, health officials said.
The victims are still being identified. Sheriff Israel said a football coach was among the dead but no names have been released.
How the attack unfolded
On Wednesday, the local public school district tweeted that "students and staff heard what sounded like gunfire" just before the end of the day at the Marjory Stoneman Douglas High School.
The attack began at 14:30 local time (19:30 GMT).
Witnesses said that the suspect set off a fire alarm before the violence erupted.
Police and Swat team members swarmed the campus and began clearing students from the school in Parkland, about an hour north of Miami, as parents and ambulances converged on the scene.
Mr. Cruz, who had been expelled for "disciplinary reasons", was taken into custody without incident in the nearby town of Coral Springs about an hour after he left the high school, according to police.
"We have already begun to dissect his websites and things on social media that he was on and some of the things... are very, very disturbing," Sheriff Israel said.
"Just a horrible day for us," said Robert Runcie, the superintendent of the county's school district.
The school has nearly 3,000 students. Classes are canceled for the rest of this week.