Two teenage gunmen opened fire at the Islamic Center of San Diego on Monday, killing three men outside the mosque, including a security guard.
The suspects, aged 17 and 19, were later found dead from apparent self-inflicted gunshot wounds, police said.
San Diego Police Chief Scott Wahl said the FBI is assisting in the investigation, which is being treated as a hate crime.
All children at the mosque's day school were safe. A separate shooting at a landscaper blocks away is under investigation.
