Police said emergency response teams found the victims outside the sprawling Islamic Centre of San Diego, before later finding the shooters, aged 19 and 17, also dead.
TV footage from a helicopter showed armed response teams gathered outside a building, with one unidentified person lying in a pool of blood.
Dozens of patrol cars were lined up around the Islamic Centre, described on its website as the largest mosque in San Diego county, which lies in southern California.
After a short period of lockdown when authorities advised residents to stay inside, San Diego police announced that the threat at the centre had been “neutralised.”
“We received a call of an active shooter at the Islamic centre. Within four minutes, officers arrived on scene and observed immediately three deceased victims out in front,” San Diego Police Chief Scott Wahl told reporters.
“We immediately began to deploy with an active shooter response into the mosque and adjacent school,” he said, adding that police had received calls about more gunfire nearby, where a landscaper working there had been shot at but not hit.
Outside the Islamic centre, police found a vehicle in the middle of the street with the shooters dead inside.
“The suspects at this point appear to have died from self-inflicted gunshot wounds. There were no officers involved in firing their weapons,” Wahl said.
Police said that a security guard at the Islamic centre was among the three victims killed. The identity of the other two fatalities was not immediately clear.
The imam at the mosque, Taha Hassane, said that all the staff, teachers and children at the mosque were safe.
“We have never experienced tragedy like this before. And at this moment all that I can say is, sending our prayers and standing in solidarity with all the families in our community here,” he said.
“It is extremely outrageous to target a place of worship,” the imam added.
US President Donald Trump described the shooting as a “terrible situation.”
“I’ve been given some early updates but we’re going to be going back and looking at it very strongly,” he told reporters.
State Governor Gavin Newsom was briefed on the apparent attack, his press office said.
“We are grateful to the first responders on the scene working to protect the community and urge everyone to follow guidance from local authorities,” it said in a post on X. (AFP)
