Firefighters said there had been about 50 people inside the building when the tragedy happened in Oakland's Fruitvale district late on Friday night.
Alameda County Sheriff's Office spokesman Sgt Ray Kelly said the victims are believed to be people aged in their 20s.
He added that the coroner was preparing for a "mass casualty event" that could also include victims from other countries.
Crews saw no evidence that smoke detectors were activated at the electronic music event and there was no sprinkler system in the building, chief Deloach Reed was quoted as saying.
