Several victims were in surgery late Sunday and others were still critical late Sunday, Zimmerman said.
One man was taken to the hospital after he broke his arm while fleeing.
“This is truly a horrific act of violence that took place here today,” San Diego Mayor Kevin Faulconer said at a news conference. “Our entire city, all of our thoughts and prayers, all San Diegans’ thoughts and prayers, are with the victims and their families tonight.”
A police helicopter arrived to the scene first and the pilot reported seeing multiple victims and that the suspect, Peter Selis, was still in the pool area and appeared to be reloading.
Three officers on the ground shot the suspect after he pointed a large-caliber hand gun at them, Zimmerman said.
Police believe that Selis lived in the complex but have not discovered a motive.
One of the partygoers is believed to live at the same complex, Zimmerman said.
Police are investigating.