According to authorities, the fire started at 11:30PM local time on December 2.
The fire became so out of control that at one point all fire units were ordered out of the building.
The venue was hosting a concert by electronic group Golden Donna as part of its 100% Silk tour.
Local broadcasters said some 13 other people were missing. The KTVU network said about 50 people were inside.
The building housed artists in studios, it said. It did not appear to be a formal concert venue.
One of the artists who lived in the building, photographer Bob Mule, told the East Bay Times he tried but failed to help a friend get out.
“I literally felt my skin peeling and my lungs being suffocated by smoke,” he said.
“I couldn’t get the fire extinguisher to work.”
Fire chief Teresa Deloach-Read told the newspaper the building had no sprinkler system.
Videos posted on social media by the fire department showed a large fire burning through the roof of the building. Three fire engines were called out.
The event’s Facebook page said the event, that featured seven acts, was to be held in a secret location in the east of Oakland that was announced only on the day it was held.
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