From the article Cpg linked:
During an afternoon press conference, city officials said that the building owner had been cited by the city just last month and an investigation was underway into if the interior of the structure was illegal.
Officials said that the last use of the property was legalized to be just a warehouse and not a residential building, as the city was investigating reports that people were living inside the warehouse.
The article has some interesting pix of the interior (undated) and of many missing people. The pix of the interior look like the inside of an antique store - lots of wood furniture that would impede egress for those inside and ingress for would-be rescuers, and which would be fuel for the fire. Looking at Google Maps, it looks like it was about 5 blocks from the Fruitvale BART station (
Bay
Area
Rapid
Transit) and a similar distance from I-880 (the Nimitz Freeway). Close by on the other side of the Nimitz is the island of Alameda. It looks like the immediate neighborhood around the BART station has some 1970s or 1980s construction (or rehab) retail business buildings, but this warehouse was 2 or 3 blocks from that area.
Driving to and from several races has taken me through that area on the Nimitz several times in the last couple of years. Much of it is former industrial buildings, abandoned in the 60s or 70s (if not the 50s). So these buildings would be older construction, of questionable or marginal compliance to current codes for
industrial use, and possibly having decades of wiring that was done by users without permits and isn't code compliant.
Even if that

"artists' collective"

had maintained the building perfectly for their business, it was not suitable for habitation, and especially not for use as an entertainment venue.