Police are responding to reports of an "active shooter" described as a teen boy at a high school in Santa Clarita, near Los Angeles.
US reports say 'at least seven' people have been shot and Saugus High School is in lockdown.
The LA Times reports police say they are searching for a suspect described as a 15-year-old boy wearing a black hat and black clothing.
Footage of the scene shows students with their hands raised being escorted by police officers from the school, which has a roll of around 2,300 pupils.
The LA County Sheriffs office confirmed on Twitter officers are responding to a shooting at the school.
Police have put several neighbouring schools under lockdown as they search for the shooter.
NBC Los Angeles earlier reported at least seven have been shot, including one victim found in a choir room and others found in a nurse's office.
Police are yet to confirm the number of casualties but local media have reported up to seven people have been shot.
The condition of the victims has yet to be confirmed by official sources.
The Sherriff's officials tweeted that no arrests had been made yet as they warn residents of neighbourhoods surrounding the school to lock themselves indoors.
Police said the suspect has been described as an Asian male wearing black clothing.
Undersheriff Tim Murakami tweeted: "Shooter at Saugus HS. Several injured. LASD resources on site and searching for suspect. Will be locking down area schools. Advise residents to shelter in place and report any suspicious activity."
The county sheriffs advised police are locking down all schools in the William S Hart District as a precaution.
Residents near the school were urged to stay inside and lock their doors and report any sightings of males matching the suspect's description.
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