The death toll from a fire at a California warehouse where a rave party was taking place rose to 33 on Sunday, with the authorities saying they had no idea how many more bodies they may find.
"When we started this investigation, if you would have told us we would have 33 victims, we wouldn't have believed you," Sergeant Ray Kelly of the Alameda County Sheriff's Department told reporters, adding that crews had searched less than half of the gutted building.
"I don't know how many people are left in there. We have no idea how many people were in that building that night... We're expecting the worst and hoping for the best in regards to how many more victims we find."
The fire in Oakland, near San Francisco, broke out about 11:30 pm Friday (0730 GMT Saturday) at the cluttered warehouse where artists and students worked and lived, even though the structure wasn't licensed for such use.
The electronic dance music party, with between 50 and 100 guests, also took place without a permit.
Although the cause of the fast-moving blaze remains under investigation, Oakland Mayor Libby Schaaf said the district attorney had opened a criminal investigation as a precaution to preserve options as the case unfolds.
"You have to understand that the scope of this tragedy is tremendous," she said. "We have many, many witnesses to interview. We are in the process of doing that."
Eight victims have been identified based on fingerprints so far, ranging in age from 17 to 35.
Some were from Europe and Asia, and the Oakland authorities are working with the State Department to contact foreign governments, Kelly said, declining to reveal which countries.
Bodies were found scattered throughout the warehouse, known locally as Oakland Ghost Ship.
"We're finding people throughout the entire square footage of that structure," Kelly said. "It's so random. We're finding victims where we least expect it."
The authorities on Sunday asked relatives of the missing to "eliminate future delays" in identifying victims by preserving such items as hairbrushes and toothbrushes for DNA samples.
"We will ask for them as we need them," said Captain Melanie Ditzenberger of the sheriff's department coroner's bureau.
Oakland Fire Chief Teresa Deloach Reed has said the interior of the warehouse was maze-like, "filled end-to-end with furniture, whatnot, collections."
"There wasn't a real entry or exit path," she said.
Images published online show artwork, pianos and wooden objects throughout the building, which helps explain why the blaze raced through the structure despite firefighters' arrival within three minutes.
Firefighters also said the building appeared to have had no sprinklers or smoke detectors.
Officials said the roof collapsed onto the second floor, which was connected to the ground floor only by a makeshift system of wooden pallets.
A dozen bodies were found in an area in the middle of the building, Oakland Fire Battalion Chief Melinda Drayton said.
Firefighters had taken every precaution to treat human remains with respect as they cleared debris bucket by bucket, she added.
The fire was the deadliest incident in Oakland since the 1989 Loma Prieta earthquake in northern California, which killed 63 people.
The deadliest nightclub fire in the United States in recent decades occurred in 2003, when pyrotechnic effects by the rock band Great White set off an inferno at The Station nightclub in Rhode Island, killing 100 people.
Oakland, a city of 420,000, was once deemed largely unsafe but is now home to a more affluent population attracted by affordable rents and proximity to San Francisco.