The gunman, Peter Selis, 49, shot all seven at a pool area at the La Jolla Crossroads apartments around 6 p.m. before police officers fatally shot him, Zimmerman said. Some of the injured were in critical condition late Sunday night, she said.
The upscale complex is located near the University of California San Diego campus and the shooting rang out in what is normally a tranquil location.
Enzo Zhuang, a student at the school whose apartment overlooked the pool, had been playing video games in his apartment when he heard arguing and minutes later, noises that sounded like fireworks.
"I thought to myself, who would be lighting fireworks," Zhuang said, adding,"it was always peaceful here before this happened."
Another person was injured in the incident, breaking his arm while fleeing the gunfire, police said. Zimmerman initially said eight people had been shot.
The shooting victims were identified only as four black women, two black men and a Hispanic man. Their names were not immediately released.
It's not clear whether Selis, who was a white man, knew any of the victims, police said. Detectives have not determined the motive for the shooting.
Some of the people at the pool were celebrating a birthday, police said. It's not clear exactly how many people were in the pool area at the time of the shooting.
Police said they believe Selis and at least one of the party-goers lived at the apartment complex.
"This was a truly horrific and disturbing act. We pray for the victims and thank our first responders. Our city rejects this senseless violence," San Diego Mayor Kevin Faulconer said at a news conference.
'People are shocked'
As police responded to 911 calls, officers flying above the complex in a helicopter could see the suspect in the pool area, and he appeared to be reloading a large-caliber handgun, authorities said.
Three officers on the ground confronted the shooter, who pointed his weapon at them, prompting an exchange of gunfire, police said.
A crowd gathers outside the La Jolla Crossroads apartments in San Diego.
In a span of about 30 minutes, apartment residents heard gunfire, sirens and the screams of those near the main pool, said resident Susan Berry, who was at the property but did not witness the shooting.
Video from the scene showed ambulances, yellow police tape and a heavy police presence. Berry said these things aren't a common sight at the complex, which college students, physicians and military families, among others, call home.
"People are shocked because it's an affluent neighborhood," Berry said.
CNN's Paul Vercammen reported from San Diego, with Nicole Chavez writing in Atlanta. CNN's Joe Sutton and Joe Sterling