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-calibre hand gun at them, Zimmerman said.
Police believe 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.