9 Dead, 'A Couple Dozen' Unaccounted for in Fire at Oakland Warehouse: Officials

— -- Authorities have confirmed nine fatalities from a fire that swept through a warehouse in Oakland, California, where a party was taking place late Friday night, but the death toll is expected to rise as firefighters worked overnight to secure the space so they may remove the bodies of other victims.

Multiple bodies "have been seen but have not yet been reached," because the building's unstable structure has prevented firefighters from removing the bodies, Alameda County Sheriff Sgt. Ray Kelly said Saturday, adding that "a couple of dozen" of party-goers remain unaccounted.

Heavy equipment, including excavators arrived overnight to begin digging out debris. Kelly said the roof of the building had collapsed and firefighters were shoring up the charred structure to ensure it is safe before recovery crews can enter.

Kelly said authorities are prepared for a mass causality event and they will be on scene for days to come. Most of the victims were in their 20s and 30s, and some were not from the U.S., he said.

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, the fire chief added.

"We are also working with federal partners to determine the cause of the fire and assist with the investigation moving forward," Reed said in a press release Saturday afternoon. "Due to structural damage from the fire, the ongoing investigation efforts and search for additional victims will be methodical and are anticipated to take some time."

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 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 that the building is home to about 18 people who use it as an artist collective.

Officials told KGO 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.

Meanwhile, a vigil had been planned for Monday evening at The Pergola at Lake Merritt in Oakland. According to the event's Facebook page, more than 800 people have RSVPed.

And a relief fund set up by the Gray Area Foundation for the Arts, Fire Relief Fund for Victims of Ghostship Oakland Fire, has already raised more than $100,000 as of Sunday morning.

ABC News' Devin Villacis, David Caplan, Rex Sakamoto, Brendan Rand, Maia Davis, Matt Foster, William Gretsky, Marilyn Heck, Josh Margolin, Troy McMullen, Alyssa Pone and Benjamin Stein contributed to this report.