"We have confirmed that the count of the deceased at 30. That is an astronomical number," Sergeant Ray Kelly of the Alameda County Sheriff's Department told reporters.
Authorities have said they fear the death toll could climb to 40 as they search the remnants of the converted two-story warehouse located near San Francisco in the city of Oakland.
"We are still not done... they are working and pulling this building apart and dissecting it. And so we're really starting to get deeper into the building. As we do that, we continue to find more victims," Kelly said.
The building had been used by artists as a living and work space but had no license for such use, officials said, nor for the electronic dance party under way when the blaze broke out late Friday.
Authorities had previously said that nine people had died in the fire, whose cause is still unknown.
Only three families of people missing in the blaze have so far been notified of the death of a loved one due to the arduous identification process, Kelly said.
Firefighters worked for 12 hours by night to enter the building, which they carried out by breaching a wall then clearing their way into the interior, said Oakland Fire Chief Melinda Drayton.
