Police launched a manhunt on Wednesday for a shooter who opened fire at a high school in Florida, reportedly leaving between 20 and 50 people injured, officials said.
The shooting took place at Marjory Stoneman Douglas High School in Parkland, a city in the southern part of the state between Boca Raton and Fort Lauderdale.
The Margate Fire Department told CBS Miami there were between 20 and 50 victims. The extent of the injuries was unclear.
"AVOID THE AREA," the Broward County sheriff's office said on Twitter.
"There are reports of victims," the sheriff's office said. "Shooter still at large."
Television images showed students being led out of the school by heavily armed police officers and an armoured vehicle filled with a SWAT team arriving at the scene.
One injured victim was seen being placed into an ambulance on a stretcher.
Police officers in helmets, bulletproof vests and armed with automatic weapons could be seen stationed at several points around the school.
"What happened was that the fire bell rang and we all thought it was a drill because we had already had a fire drill," an unidentified student told WSVN 7 News.
"We thought it's another drill so we didn't take it seriously and then we heard pops from the other side of the school," the student said.
Police from neighbouring Coral Springs advised teachers and students in the building to "remain barricaded inside until police reach you".
A local television reporter he had been in touch via text message with one female student who told him she had taken refuge in a closet with a number of classmates.
The teen told the reporter that she had heard a noise that "sounded like a bomb", and a drama teacher had then told the pupils to huddle inside the closet.
"Everyone inside is very scared, it's very warm," the student was quoted as saying.
"My prayers and condolences to the families of the victims of the terrible Florida shooting," President Donald Trump said on Twitter.
"No child, teacher or anyone else should ever feel unsafe in an American school."
Marjory Stoneman Douglas High School, part of the Broward County public school district, has nearly 3,000 students, according to 2014 figures.
The shooting, one of several since the start of the year, will once again throw the spotlight on America's epidemic of gun violence and the ready accessibility of weapons, with 33,000 gun-related deaths annually.
Since January 2013, "there have been at least 283 school shootings across the country — which averages out to one school shooting a week", according to Everytown for Gun Safety, a non-profit group that advocates for gun control.
Since the 2012 massacre at the Sandy Hook Elementary School in Newtown, Connecticut, where 20 children and six adults were shot dead, warning procedures and emergency drills have multiplied at US schools.
The goal is to teach school children how to react to a shooter who opens fire at random.