The shooting took place in the car park of Fairfield High School in the city of Fairfield, approximately 80 kilometres northwest of San Francisco.

According to police, officers were alerted to the incident at around 7:15 pm local time after gunfire broke out as attendees gathered in the car park following the ceremony.

One dead, three wounded

Authorities confirmed that an 18-year-old was killed in the shooting. Three others, aged 11, 20 and 25, sustained gunshot wounds and were taken for treatment.

Speaking at a news conference, Fairfield Police officer Michelle Belyea said there were four victims with gunshot wounds.

Witnesses describe panic

A man attending the graduation ceremony told local media he heard several shots as families and graduates were taking photographs. Nearby resident Amanda Prieto said she saw people screaming and running through the car park as the gunfire erupted.

Investigation underway

Police said the shooting remains under active investigation. Authorities have not released details about a suspect or the condition of the injured victims, but have confirmed there is no ongoing threat to the community.