The fire that killed 36 people during a party at an Oakland warehouse started on the ground floor and quickly raged, with smoke billowing into the second level and trapping victims whose only escape route was through the flames, federal investigators said Wednesday.

"The occupants were consumed by smoke before they could get out of the building," said Jill Snyder, special agent in charge of the San Francisco office of the Bureau of Alcohol, Tobacco, Firearms and Explosives.

She also confirmed there were no sprinklers or fire alarm system in the building.

Officials previously said recovery efforts at the site had ended with the death toll remaining at 36.

Snyder reiterated that investigators were examining a refrigerator and other possibilities as a potential source of the fire but had not reached any conclusions. Investigators have not found evidence of arson, but she said they have also not ruled it out.

Snyder did not give a timeline for the completion of the investigation.

Officials hope to create a virtual, 3D reconstruction of the building for criminal and civil investigations and to provide answers to family members about their loved ones' last moments, Alameda County sheriff's Sgt. Ray Kelly said.

"Where was my son or daughter? Where were they standing? How come they didn't get out?" he said. "Families want answers. They need answers."

The fire broke out during a dance party Friday night in the cluttered warehouse.

The structure had been converted to artists' studios and illegal living spaces, and former denizens said it was a death trap of piled wood, furniture, snaking electrical cords and only two exits.

Danielle Boudreaux, a 40-year-old hairdresser who was close with Derick Ion Almena, the founder of the artists' colony, and his partner, Micah Allison, said it was full of extension cords plugged into each other that helped supply power to music equipment, microwaves and hot plates.

There were also RVs inside, she said.

Almena has not responded to emails or calls by The Associated Press to phone numbers associated with him. He has said he didn't attend the event Friday night, and he is sorry.

City and state officials fielded complaints for years about dangerous conditions, drugs, neglected children, trash, thefts and squabbles at the warehouse, raising questions about why it wasn't shut down. The district attorney has warned of possible murder charges as she determines whether there were any crimes linked to the blaze.

The fire has cast a spotlight on similar artists' colonies around the San Francisco Bay Area that offer cheap housing in one of the country's priciest rental markets but have raised safety concerns among neighbors and city officials.

Oakland Mayor Libby Schaaf said the city will look to strengthen regulations for smoke alarms and exits and clarify city employees' responsibilities to monitor unsafe structures.

She said the city has been flooded with complaints about building safety since the fire and she wants to engage the arts community in the review to ensure any changes don't jeopardize their access to affordable housing and work space.

On Wednesday, an Oakland restaurant owner, Dorothy King, called a news conference to say she was concerned the artists' warehouse next to her business could pose a safety hazard.

Several artists tried to shout her down, accusing her of launching a "witch hunt" that could result in artists' collectives being shut down.

King said that wasn't her intention.

"I'm concerned about my business burning down. I'm concerned about a tragedy that could happen," she said. "The city should come in and help, not shut it down."