At least one person was killed and three were injured when a gunman wearing black clothing opened fire in the grounds of a high school in California, officials say.
The shooting took place at the Saugus High School in Santa Clarita, north of Los Angeles, minutes before the school day was due to begin.
Police say a suspect is now in custody.
Saugus High School and other neighbouring schools have been placed on lockdown.
According to the LA Times website, the suspect is a 15-year-old boy.
Live video from helicopter showed officers swarming the school, and injured victims being placed in ambulances.
Henry Mayo Hospital said that one female patient had died. Two male patients were in critical condition, with a third in good condition.
The LA county sheriff said the suspect was being treated in a local hospital.
Many US schools have implemented active shooter drills in recent years, following a string of attacks.
According to the Washington Post, more than 230,000 young people have experienced gun violence at school since the Columbine High School massacre in Colorado in 1999.