moodboard/Thinkstock(OAKLAND, Calif.) — Authorities fear there may be as many as 40 fatalities from a fire Friday night at a warehouse in Oakland, California, where a party was taking place, though at least nine deaths have been confirmed thus far, officials said.

Sgt. Ray Kelly of the Alameda County Sheriff’s Office said authorities are prepared for a mass causality event.

During a press conference Saturday, Kelly said the roof of the building had collapsed and firefighters were shoring up the charred structure to ensure it’s safe before recovery crews can enter.

The Oakland Fire Department responded to reports of a structure fire on 31st Avenue at around 11:32 p.m. on Friday night. Most of the bodies were found on the warehouse’s second floor, which was only accessible by a makeshift stairwell assembled with various materials, according to Oakland Fire Chief Teresa Deloach Reed.

Reed said the warehouse appears to be a work and residential building that hosts a makeshift artists’ studio. It’s unclear if the building is equipped with smoke alarms, but officials said there did not appear to be sprinklers.

Aside from those confirmed dead, many are still unaccounted for and more fatalities are expected. A thorough search of the building and recovery effort is still underway, authorities said Saturday.

An official briefed on the ongoing investigation said there were stacks of wooden pallets inside the warehouse that essentially served as kindling for the fire.

Preliminary indications suggest the blaze was not caused by arson, but rather was an electrical fire. The power went out inside the building when the fire began and the flames blocked the building’s only exit, making it difficult for people inside to escape, the official told ABC News.

Investigators will look into the building’s inspection history and will try to determine whether the owner was aware what the structure was being used for. Criminal charges could be possible depending on the outcome of the investigation, the official said.

One person who was inside the warehouse at the time of the fire told ABC affiliate KGO-TV that the building is home to about 18 people who use it as an artist collective.

Officials told KGO-TV there was an event with at least 75 people inside when the flames erupted Friday night.

Footage from the scene showed the building engulfed in flames as plumes of thick, grey smoke billowed into the dark sky.

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