The building, known locally as the Ghost Ship, was being used as an artists' space, though officials said it was zoned for use as a warehouse. Some neighbors and former residents said people lived there.
Here is what we know:
Had there been complaints about conditions at the building?
Oakland City Councilman Noel Gallo, who represents the district where the fire broke out, said he had been hearing complaints from constituents about debris outside the warehouse. Asked if the building had permits for people to live there, he said: "Absolutely not."
"The reality is, there are many facilities being occupied without permits," he said. "They're occurring on Oakland's streets, especially in neighborhoods like mine."
Neighbors had repeatedly contacted the city about trash and debris piled up outside the warehouse. On Nov. 14, inspectors with Oakland's Planning and Building Department began investigating an allegation of illegal construction inside the building.
Darin Ranelletti, the department's interim director, said inspectors attempted to enter the warehouse three days later but failed.
Ranelletti said his department had also received reports that people were living in the warehouse illegally. Inspectors were still investigating that allegation before the fire broke out, he said.
Who owns the building?
The Oakland warehouse is one of several properties owned by Chor N. Ng, according to her daughter, Eva Ng, 36. She was adamant that the warehouse was being leased as studio space for an art collective and not used as residences.
Eva Ng said she had been reassured by the lease holder that nobody lived in the building. "They confirmed multiple times. They said sometimes some people worked through the night, but that is all," Ng said.
Still, one former tenant told The Times that he remembered at least 10 people living inside the warehouse. Bradley Evans, who moved out in August 2015, said the man who collected the rent lived in the building with his wife and three children.
The man instructed tenants to tell the landlord they were working on art projects and not living there full time, said Evans, 21. The landlord "came by once a month to collect the money and didn't ask any questions," he added.
Ng said she believed the building had smoke detectors and two second-floor exits, both wooden stairs. She was not familiar with a report that one of the stairs was at least partly made of pallets. But she was aware that there had been parties in the building.
"It's Christmas, after all," she said.
How do former residents and visitors describe the building?
People who previously lived there recalled a building that lacked fire sprinklers and had a staircase partly made of wooden pallets. Partygoers described a rabbit warren of rooms crammed with belongings — pianos, organs, antique furniture, doors and half-finished sculptures.
"It was a tinderbox," said Brooke Rollo, 30, who lives less than a mile from the scene and had gone to parties there.
Firefighters who responded to Friday's three alarm blaze described the interior as a labyrinth.
Was the concert permitted?
City officials said no one got a permit to hold the concert at the building.
Cities in California typically require a special permit for one-time events — such as weddings, art openings and concerts — in buildings not constructed for such activities, said development consultant Hamid Behdad, who supervised L.A.'s effort to convert commercial buildings to lofts and apartments.
To secure those permits, an owner must show fire and building inspectors where the exits are, the locations of the fire extinguishers and how the building will be illuminated in case of an emergency, he said.