A gunman has shot dead one worshipper and left three others injured in hospital after opening fire at a synagogue outside San Diego.
A 19-year-old man was arrested at the Chabad of Poway Synagogue, but police have boosted patrols at other places of worship as a precaution.
The extent of the victims injuries is unclear, officials said.
The incident came exactly six months after a shooting at a Pittsburgh synagogue killed 11 people in the deadliest attack on Jews in US history.
Officials said San Diego County deputies were called just before 11.30am local time.