The roof of a two-story warehouse in the city's Fruitvale district collapsed during the fire, complicating efforts to recover bodies.
Fire officials were still trying to determine how the blaze started at about 11:30 p.m. on Friday, said Chief Teresa Deloach-Reed.
The roof of the two-story warehouse in the city's Fruitvale district collapsed during the fire, complicating efforts to recover bodies, she told a press briefing.
Officials did not know if any of the 25 missing people were among the nine confirmed victims.
"There is a large majority of that building that has not been searched," Deloach-Reed said during a press briefing.
"We are hoping that the number nine is what there is and that there are no more," the fire chief said, referring to the number of known fatalities.
Deloach-Reed said some of those who were missing may have brought themselves to the hospital or elsewhere. She said she did not know how many people were at the party.
The warehouse housed units where people lived and worked - makeshift artist studios carved out with partitions, the fire chief said. "A flea market of items" were inside, she said.
A Facebook event page showed 176 people planned to attend the party, which featured a performance by the electronic music act Golden Donna. The page, which listed 355 others as interested in going, carried posts from people who were either missing or accounted for.
Video footage posted on social media showed the structure engulfed in flames and encircled by fire vehicles pumping water into the building.
"It's going to hit the city, it's going to hit our organization," Reed said. "It's just going to be hard on everyone."