Officials said people either escaped from the cluttered building or died inside, where the only way down from the second story was via a stairwell constructed entirely of wooden pallets.
"It appears that either you got out or you got trapped inside," said Alameda County Sheriff's Office spokesman Sgt. Ray Kelly.
Just last month, the city of Oakland opened an investigation into the warehouse after numerous complaints filed by neighbors complaining of trash piling up outside the property and reports people were illegally living in the building, which was zoned as a warehouse.
Darin Ranelletti, of the Oakland Planning Department, said the city opened an investigation on Nov. 13 and an investigator went to the premises on Nov. 17 but could not get inside the building. The city has not confirmed people were living inside.
Firefighters used chain saws and axes Saturday to cut through the debris of the cluttered building in a gritty neighborhood of the San Francisco Bay Area city, where the fire broke out about 11:30 p.m. Friday.
Aerial images of the warehouse showed the structure was gutted with just the wooden frame remaining. Fire officials said the roof collapsed into the second floor, which in places fell to the bottom floor.
It's unclear what sparked the fire.
Oakland fire chief Teresa Deloche-Reed said at least another 25 people were unaccounted for in what authorities were calling the most deadly building fire in the city in recent memory. The victims were believed to be young people in their 20s, Kelly said. He said as many as 40 may have perished and that the coroner is preparing for a "mass casualty event" that could include victims from other countries.
The warehouse was known as the "Oakland Ghost Ship." Its website showed pictures of a bohemian, loft-like interior made of wood and cluttered with rugs, old sofas and a garage-sale like collection of pianos, paintings, turntables, statues and other items.
The website included advertisements for various electronic music parties. On Friday night, there was an event featuring musician Golden Donna's 100% Silk West Coast tour.
Panicked friends and family posted messages on the group's Facebook page trying to find out if their loved ones were among the dead. Those searching for the missing were sent to a local sheriff's office, where Dan Vega was anxiously awaiting news. He had been unable to find his younger brother or his brother's girlfriend. Vega said he was not sure if the two were at the party Friday night but that his brother likes to go to raves and he had not been able to reach him Saturday. His girlfriend's car was still parked at a transit station in San Bruno, south of San Francisco.
Fighting tears, Dan Vega said he's frustrated authorities hadn't been able to tell him anything about his 22-year-old brother.
"I just want to go over there. I have my work boots on, I'm ready to go," Dan Vega said. "Just give me some gloves. I'll help out any way, shape or form, I don't care. This is infuriating. I don't know where my brother's at. I just want to find him."
It was not immediately clear what started the fire, but there did not appear to be sprinklers in the building, Deloche-Reed said. She said 50 to 100 people were believed to have been at the party when the fire started and that clutter "made it difficult for people to escape."
The warehouse was partitioned into artist studios and was packed with furniture, mannequins, statues, lamps, and other objects and did not have a clear entry or exit path, the fire chief said.
"There is still a large portion of the building that still needs to be searched," she said. "There is large timber and debris that will need to be removed and it's going to have to be removed in a slow and methodical way."
By midday Saturday, authorities had not yet identified the nine dead. Kelly said crews had removed only one body from the building.
Investigators were having trouble entering parts of the warehouse to search for any remaining bodies because the structure was deemed unsafe, Deloche-Reed said.
One survivor said he struggled to find a working fire extinguisher.
Nine dead and 25 unaccounted for at warehouse fire in Oakland. People lived in warehouse that was hard to access. pic.twitter.com/g5Sa7qr1Ks— Jim Huntington (@JamesHenry54) December 3, 2016
"It was too hot, too much smoke, I had to get out of there," Bob Mule, a photographer and artist who lives at the building and suffered minor burns, told the East Bay Times. "I literally felt my skin peeling and my lungs being suffocated by smoke. I couldn't get the fire extinguisher to work."
Oakland police urged those concerned about missing people to call the Alameda County Sheriff's Coroner's Bureau at 510-382-3000.
The office said coroners were also at the scene Saturday morning and unavailable for comment.
Click here to find out if people in the area are safe.