Officers were alerted to the incident at the Islamic Center of San Diego, and found three victims with gunshot wounds outside the front of the building.
Two suspects, aged 17 and 18, were found dead of self-inflicted wounds in a vehicle a couple of blocks away.
Among the victims security guard Amin Abdullah.
Police Chief Scott Wahl said the actions of Mr Abdullah, a father of eight children, helped prevent an even deadlier attack.
“His actions were heroic and he undoubtedly saved lives today,” he said.
Lockdown: Police at the scene in San Diego
The San Diego Union-Tribune via Getty Images
What do we know about the suspects?
According to US news reports, the suspects have been identified as Cain Clark, 17, and Caleb Vazquez, 18.
Police said they received a call at 9.42am local time (5.42pm BST) about a runaway juvenile.
The juvenile’s mother believed her son was suicidal and reported that several of her weapons were missing, along with her vehicle.
She also stated that he was with a companion, who was dressed in camouflage.
San Diego Police Department used Automated License Plate Recognition (ALPR) technology and officers on the ground to try and track down the pair.
It was reported to police that one of the suspects was associated with Madison High School. He had in fact enrolled in an online virtual programme.
The New York Post is reporting that one of the suspects left a suicide note that referenced racial pride.
It says Clark attended Madison High School and was, according to the school’s social media page, a standout wrestler.
His grandfather, David Clark, 78, said: “We’re very sorry for what happened. We know as much as you do. It’s a shock.”
Clark and Velasquez were reportedly found dead inside a BMW from self-inflicted gunshot two few blocks from the Islamic Center of San Diego.
According to reports, anti-Islamic writings were found in the suspects’ vehicle, while “hate speech” was written on the firearms used in the attack.
