The Broward Sherriff's Office said it was "working a developing incident" at Stoneman Douglas High School.

"There are reports of victims," it added, warning that the shooter was still at large and urging people to avoid the area.

A local Fox news affiliate reported seeing several people being treated for injuries outside the school.

Helicopter video showed armed police at the building's perimeter.

Coral Springs Police department, meanwhile, tweeted instructions to teachers and students to "remain barricaded inside until police reach you".

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