Pablo Gutierrez heard the pop-pop over the roar of nearby Interstate 15. Fireworks, he thought at first. Then he realized: No, that’s gunfire.

When Mr. Gutierrez realized the sounds were coming from the Islamic Center of San Diego and its attached school, he raced to see if there was anything he could do to help.

“By the time I got to the school they were letting parents in to find their children,” he said.

The school and the mosque are near the interstate, but drivers can see the white building with a tall minaret rising into the sky if they look to the west.

After the shooting, reporters and officials from the F.B.I., the police and the fire department gathered at a park a block from the mosque. The building’s entrance was taped off and quiet, but through the gates a body could be seen, lying on a walk beneath a black covering.

A helicopter flew overhead.

“My prayers are with you and we’re going to do everything it takes to keep this community safe,” said San Diego’s mayor, Todd Gloria, looking as though he was on the verge of tears.

The violence had jarred an idyllic corner of Southern California, shaded by palm and eucalyptus trees, with purple jacarandas in full bloom.

“I’ve lived here most of my life, and this is a quiet neighborhood,” said Robert Benton, 64. He said he lives less than a mile from the mosque and a synagogue.

“There’s always security at the synagogue, but I wasn’t sure about the mosque,” Mr. Benton said. “It’s a sad world that we live in that we have to have security guards at places of worship.”