Firefighters in Oakland, east of San Francisco, found the remains of nearly three dozen victims as they sifted through the debris-filled shell of the two-story converted warehouse being used by an artists' collective.

The cause of the fire was still undetermined, officials said. Arson is not suspected but investigators want to find out if the building, often used for musical performances, had a history of code violations.

Mayor Libby Schaaf said the Alameda County District Attorney's Office activated its criminal investigation team at the fire scene. A representative of the prosecutor's office is monitoring the recovery process, she said, adding she was not authorised to say if a criminal probe was under way.

The mayor said the city's first priority was finding the victims and supporting the families. "We have delivered the unacceptable and horrific news of losing a loved one to seven of our families."

The blaze, which erupted about 11:30 p.m. on Friday (0730 GMT on Saturday), ranks as the deadliest in the United States since 100 people perished in a 2003 Rhode Island nightclub fire.

The warehouse, which served as a base for the Ghost Ship Artists Collective, was one of many converted lofts in the city's Fruitvale district, a mostly Latino area where rents are generally lower than in the rest of Oakland.

As of Sunday evening, only 35 to 40 percent of the building had been searched, said Sergeant Ray Kelly, spokesman for the Alameda County Sheriff's Office.

Officials are still unsure how many people were in the building at the time of the fire and how many more victims might be found.

SOME VICTIMS AGED 17 OR YOUNGER

Recovery teams had yet to search an unspecified number of mobile homes parked on the first floor, Kelly said, in illustrating the scope of the task. He said the trailers appeared to have people living in them.

The building was designated for use as a warehouse only, according to the city. The city was aware of reports that people were living there, but no permits had been issued for that purpose.

The recovery operation was delayed for hours as the roof collapsed onto the second floor and in some spots, the second story had fallen onto the first, making it unsafe to enter.

The effort has proceeded slowly as teams have struggled to clear mountains of debris and locate victims who appeared to scatter throughout the unstable structure.

"We're finding victims in every quadrant of the warehouse," Kelly said. "We're finding victims where we least expect them."

Exhaustion and the scope of the disaster were taking an emotional toll on crews who had been working around the clock since the fire ignited about 36 hours earlier, officials said.

With many victims burned beyond recognition, family members were asked to preserve items that might contain DNA of the victims to help with identification.

After notifying their families, the Alameda County coroner released the names of seven victims who had been positively identified.

Some of the victims were 17 years old or younger, although most were in their 20s and 30s, officials said. Some were from other parts of the country and outside the United States.

One of the dead was the son of a sheriff's deputy, Kelly said. "This tragedy has hit very close to home," he said.

Chris Nechodom, 30, said he was on the ground floor of the building when he saw flames race across the ceiling. As he fled, he heard a loud noise and saw a plume of thick black smoke billowing out.

"It blew out maybe 10 feet out of the entrance. After that, I saw a few more people crawl out," he said. Nechodom said he was unsure how many people were inside when the fire started.

'SET UP FOR A FIRE'

Photos of the “Ghost Ship” venue posted online showed a space brimming with an elaborate array of musical instruments, religious statues and antiques. The area was furnished with a mix of overstuffed sofas and colorful carpets and featured a maze of side rooms and nooks.

"The whole place was built like you are going to set up for a fire," said Matt Hummel, 46, who twice visited the space before the fire and had helped renovate other warehouse spaces for artists.

The party took place on the second floor of the building, which appeared to have only two exits, officials said. There was no evidence of smoke detectors or sprinklers, they added.

The city had received complaints about "blight" and construction without permits and opened an investigation. An inspector verified the "blight" complaint after observing piles of debris outside, but failed to gain access to verify the construction complaint, the city has said.

Schaaf told reporters she did not know why inspectors were unable to get into the building when they tried to inspect it. She said she was putting together a team of city employees "to gather every piece of information."