At least nine people were killed and more than a dozen others were unaccounted for after the worst fire in decades in Oakland tore through a makeshift collection of artists’ spaces packed with revelers attending a late-night music party, authorities said Saturday.

The blaze ignited shortly before 11:30 p.m. Friday in the two-story converted warehouse at 1305 31st Ave. at International Boulevard, in the Fruitvale neighborhood, while as many as 100 people were inside for a performance by the Golden Donna 100 Percent Silk touring electronic dance music show.

Authorities said they expected the death toll to rise, perhaps to as high as 24.

The building had permits for use only as a warehouse, not as a living or entertainment space, and there was no evidence the structure had fire sprinklers, officials said. Known as the Ghost Ship, the complex was under investigation by city officials for allegedly unpermitted construction and blight.

Firefighters said some victims were apparently trapped when they couldn’t flee down a narrow, ramshackle stairwell leading to the second floor that was nailed together out of pallets and other bits of wood. Some victims’ bodies were believed to be in hard-to-reach corners of the unstable building, and recovering all of them could take days, authorities said.

“There’s still a lot of the building that needs to be searched,” Fire Chief Teresa Deloach Reed said at the scene.

The cause of the fire was not immediately known.

Sgt. Ray Kelly, a spokesman for the Alameda County Sheriff’s Office, said most of the victims were in their 20s or 30s. “We did not have a lot of victims go to hospitals,” he said. “It appeared they made it out, or didn’t make it out.”

The Sheriff’s Office flew heat-seeking drones over the rubble early Saturday searching for survivors, Kelly said, but didn’t find any. He said the coroner’s office was prepared to handle as many as 40 bodies.

As the fire engulfed the building north of Interstate 880, people streamed into the street outside. Scores of firefighters attacked the blaze, but when teams pushed into the building, the flames were so intense they had to back out again, said Deputy Fire Chief Mark Hoffmann.

“We went in about 20 yards on the ground floor” but were repulsed by the heat, smoke and danger from the indoor hodgepodge of hand-built furniture and artist work spaces, Hoffmann said. “It was a labyrinth.”

When the roof collapsed all the way to the first floor, all hope of quickly getting deep inside was lost.

Seven corpses were found on the building’s second level and two on the ground floor. By evening, all nine known victims had been removed from the building, and crews were carefully picking through the debris with excavators and other heavy equipment to search for more. None of the victims’ names was released.

Al Garcia, owner of Reed Supply Co. across the street from the burned building, said his sister lives near the warehouse and called him around midnight to tell him about the inferno. When he arrived, he said, flames were shooting out of the windows and roof.

“I knew people were dead,” he said. “There was no way anyone could get out.”

He found two young men, ages 17 and 18, sitting in the doorway of his business, dazed and clearly in shock. They told him they had heard about the party online and believed they were the last two people to escape the flames. They recalled rushing through thick black smoke, with the building caving in behind them.

One of the teens “heard screaming, and he followed the voices” outside, Garcia said. The businessman added, “I couldn’t sleep after hearing that.”

Terry Ewing, whose girlfriend was at the electronic dance music show and was still missing Saturday, stood stone-faced outside the Alameda County Sheriff's Office on East 12th Street, which had been converted into an assistance center for survivors and families searching for missing loved ones.

“Some friends came to my house this morning and told me there was a fire, and that we were missing some people,” he said, shaking his head. “That’s all we know so far.

“There’s a lot of family in there. People in that place were very loved.”

Joel Shanahan, the electronic musician from Madison, Wis., who performs as Golden Donna, the party’s headline act, was confirmed to be safe by associates who posted the news on social media. Local musicians also performed on the bill, and some of them were still being sought by friends Saturday.

Mayor Libby Schaaf went to the scene and, between heavy sighs, called the fire “a terrible tragedy” and promised a “thorough and methodical investigation.” She said the main focus for now was on recovering victims and consoling survivors.

“I met with a roomful of people who had loved ones that are missing,” Schaaf said. “It is painful to tell them it will be a considerable amount of time before we get the information they deserve.”

Gov. Jerry Brown, a former Oakland mayor, said he and his wife “were saddened to learn of last night’s tragic fire in Oakland. Our thoughts are with the entire city in this difficult time, and we extend our condolences to the family and friends of those lost.”

By early afternoon, a makeshift memorial with flowers appeared nearby on a fence at 12th Street and 31st Avenue, near the fire scene. Monica Rocha, 25, lives nearby and came to hang flowers. She said she had visited the building a year ago with a friend.

“It was weird because we were walking through it with my friend and we were like, ‘This is a tinderbox.’ ... We were like, ‘This isn’t safe. This could go down so easily,’” Rocha said.

She described the building as a “maze of wooden things and mannequins — it looked like a madhouse.”

A large, gray-walled warehouse with a skull and other ornate artwork painted outside, the Ghost Ship was filled with personalized, hand-built spaces tailored for musicians and artisans. Witnesses described a fantastical interior of tapestries, instruments and ornately carved ceiling and room structures.

Garcia, the neighboring businessman, said he had worried the place was a fire hazard because of the junk and debris around. The people who lived there, he said, had been trying to clean it up, painting the facade and adding the words “Ghost Ship” across the front.

City Councilman Noel Gallo, who represents the Fruitvale district and lives a block from where the fire occurred, said the building “has been an issue for a number of years.”

“People have been living inside, and the neighbors have complained about it,” he said. “Some of these young people that were in there were underage. They frequently had parties there.”

Darin Ranelletti, Oakland’s planning and building director, said officials “had received recent complaints about blight and unpermitted construction” at the warehouse and had opened an investigation Nov. 13.

North of the warehouse at Eli’s Mile High Club, a dozen musicians who were friends of performers at the show or members of the audience gathered Saturday to wait and mourn. They paced, staring at their phones and wiping tears, hoping to hear from people they believed had been at the Ghost Ship event.

“There are people unaccounted for; they’re just not responding,” said Billy Agan, owner of Eli’s. He heard of the fire soon after it broke out and said he hadn’t slept all night.

Agan said the music scene of young people in Oakland is so small that everyone knows each other.

“We’re all going to know them intimately,” Agan said of the possible victims. “It’s going to affect culture in Oakland.”

On social media, friends of the victims and the many missing partygoers desperately sought information, particularly on the Golden Donna tour site, asking for people to confirm who got out of the building — and in some cases expressing grief for those believed to be dead. Many posted descriptions of piercings and tattoos to help identify people.

“Love and strength to all. Our thoughts and hearts are with you,” posted one. “Tragic loss for our community,” posted another. Others listed names of possible victims.

Chief Deloach Reed said it was unclear how many of the more than a dozen people known to be missing might be dead. “We’re not sure if they have already self-transported to the hospital or they have gone to a friend’s home,” she said. “We’re not sure.”

The music party was scheduled from 9 p.m. Friday to 4 a.m. Saturday. A party listing detailed musical acts and DJs. It said a “secret East Oakland location” was to be announced on the day of the event. Tickets were $10 before 11 p.m. and $15 after that.

It was not immediately clear what ignited the fire or where exactly it started, Deloach Reed said. The chief said a multiagency task force was gathering to investigate the fire and take up the task of recovering victims.

“The building is a huge building,” Deloach Reed said. “There’s going to have to be a methodical way we go about body recovery, and then also trying to find out where the fire started and how the fire’s spread took place. ... We have not done a complete search of the building.”

Even with the nine confirmed deaths, the blaze was Oakland’s worst in many years. The toll could exceed that of the Oakland hills fire of October 1991, in which 25 people died.

The fire chief said the scope of Friday night’s fire was just beginning to be fathomed and that it represented a tragedy for many families.

“It’s going to hit the city, it’s going to hit our organization, to have an incident of this type, of this magnitude,” she said. “It’s just going to be hard on everyone.”

Chronicle staff writers Jill Tucker, Erin Allday, Rachel Swan and Jenna Lyons contributed to this report.

Hamed Aleaziz, Michael Cabanatuan and Kevin Fagan are San Francisco Chronicle staff writers. Email: haleaziz@sfchronicle.com, mcabanatuan@sfchronicle.com, fagan@sfchronicle.com

Finding details

on loved ones

and ways to help

Those who have information about missing people or are searching for relatives after Friday night’s fire should contact the Alameda County coroner’s office at (510) 382-3000.

Sites raising money for victims and survivors:

Gofundme: https://www.gofundme.com/ghost-ship-fire-residents-support

Oakland A’s: https://www.youcaring.com/oaklandfirevictimsfrom130531staveinoakland-706680#mlb-oakland

Gray Area Foundation for the Arts: https://www.youcaring.com/firevictimsofoaklandfiredec232016-706684