The toll in the fire at a converted warehouse in Oakland, California, has risen to 33, and authorities said it is likely to be higher as only 35% to 40% of the building has been searched so far, Reuters reported. The blaze – considered the deadliest such incident in the United States since the 2003 Rhode Island nightclub fire that killed 100 – broke out on Friday while the two-storey structure was being used by an artists’ collective for a dance party.

Fire officials have yet to determine the cause of the blaze, but they are trying to ascertain whether the warehouse had violated safety codes. Investigators are still unsure of the number of people who were inside the building when the flames spread and how many more victims may be found. Around 12 people, however, are believed to have escaped, while one other is undergoing treatment in hospital.

The operation to find the bodies was delayed for hours after the roof collapsed onto the second floor and the second storey onto the first, which made it unsafe for investigators to enter the building. “We are finding victims in every quadrant of the warehouse,” said Sergeant Ray Kelly, a spokesperson for the Alameda County Sheriff’s Office.

Some bodies have been charred beyond recognition. Family members have been asked to preserve objects that may contain the victims’ DNA to help authorities with identification. “We have delivered the unacceptable and horrific news of losing a loved one to seven of our families,” Mayor of Oakland Libby Schaaf said.