Mayor Libby Schaaf said Alameda County district attorney Nancy E O'Malley had activated a criminal investigation and the investigators were on the scene of the fire.
Schaaf said the families of seven victims had been notified and the officials would be releasing the names of the deceased "promptly".
The fire broke out around 11:30 PM Friday during an electronic music show at the "Ghost Ship," an artists' collective in Oakland's Fruitvale neighbourhood.
She also said that, since Saturday night, firefighters have only been able to search "approximately 20 per cent" of the building, describing the search as "a long and arduous process."
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"This is the most deadly fire in Oakland Fire's history that I'm aware of," she said.
Searchers have recovered 33 bodies from the fire-ravaged warehouse that housed artist studios and was hosting a dance party when the deadly blaze broke out, officials said.
Sergeant Ray Kelly of the Alameda County Sheriff's Office said the number of casualties could rise. The search effort is expected to last for days, he said.
The District Attorney's Office has criminal investigation teams actively scouring the site of the fire along with other law enforcement agencies involved in investigating the incident, according to a statement from the City of Oakland.
It's unclear if the building was equipped with smoke alarms, the fire chief said.
Oakland's warehouses have become hubs for artists and musicians in recent years.
The Oakland Fire Department said they are currently working with a variety of agencies in the aftermath of the tragedy, including the Alameda County Sheriff's Department, the Oakland Police Department, the Alameda County Search and Rescue, and the American Red Cross, among others.