Updated at 7 p.m. ET Sunday.
Officials say they have so far recovered 33 victims killed in a warehouse fire in Oakland, Calif., and say there may be more.
The fire broke out around 11:30 p.m. Friday during an electronic music show at the "Ghost Ship," an artists' collective in Oakland's Fruitvale neighborhood.
As of Sunday afternoon local time, searchers had been through "35 to 40 percent" of the building, said Sgt. Ray Kelly of the Alameda County Sheriff's Department Coroner's Bureau at a news conference.
Oakland Mayor Libby Schaaf said authorities had so far contacted seven families of victims. Kelly said they had identified victims using found IDs and fingerprints.
"I don't know how many more people are left in there," Kelly said. "We're expecting the worst and hoping for the best."
He said teenagers are among the victims, and many others are in their 20s and 30s. Foreigners from Europe and Asia are among the fire's victims.
Mayor Schaaf said that a "criminal investigation team has been activated" and is on site. The cause of the fire is still under investigation, authorities said.
"The scope of this tragedy is tremendous," the mayor said.
Firefighters were removing debris from the cluttered warehouse "literally bucket by bucket" overnight, Oakland Fire Battalion Chief Melinda Drayton said during a Sunday morning news conference. She described the search as "a long and arduous process."
"This is the most deadly fire in Oakland Fire's history that I'm aware of," she said.
The roof of the warehouse collapsed Saturday, creating safety concerns for rescuers and slowing down their progress.
"We will be here for days and days to come," said Sgt. Ray Kelly of the Alameda County Sheriff's Department Coroner's Bureau.
A musician who was at the show told the East Bay Times that as many as 40 to 50 people were in the building when it began to fill with thick smoke and became pitch black. Many people became trapped on the second floor and a makeshift staircase made escape difficult.
The Times posted a diagram of the building with photos corresponding its location. You can see it on their site.
The building is owned by Chor N. Ng of Oakland but was primarily operated by Derick Alemany and his wife Micah, according to the Times. The couple collected rent from artists in the space and held parties and concerts to raise money for rent.
"If you were not familiar with the building and the way that it was, if you were going there for a party, you wouldn't be aware of the maze that you have to go through to get out," a former friend of the couple told The Associated Press.
Posting on Facebook early Saturday, a person the Times identified as Alemany wrote:
"Confirmed. Everything I worked so hard for is gone. Blessed that my children and Micah were at a hotel safe and sound... it's as if I have awoken from a dream filled with opulence and hope.... to be standing now in poverty of self worth."
Criticism poured in on Facebook, calling his post insensitive to the people killed.
The building was zoned as a warehouse, but many people lived there illegally, according to the AP. Oakland Planning and Building Director Darin Ranelletti said during a Saturday news conference that the city had opened investigations into the property last month after complaints about "blight and unpermitted construction." He also said an investigator was unable to get into the building last month.
The electronic music show's event page was filled with sympathetic posts from well-wishers all over the world. On Saturday, friends used the page to exchange information in hopes of finding information about loved ones who were still missing. Friends created a Google document Saturday with a list of names of missing people and contact information for reaching family members and friends.
Authorities say they will soon begin releasing the names of victims who have been identified.