Oakland officials on Sunday increased the death toll of a warehouse fire to 33 people, as the mayor announced that a criminal investigation team has joined the inquiry into how an unauthorized party became one of the worst disasters in the city’s history.

At an afternoon briefing, a spokesman for the Alameda county sheriff, Sgt Ray Kelly, said 33 people had been confirmed killed. He said that investigators had only searched about 30-40% of the warehouse ruins, in Oakland’s Fruitvale neighborhood, and that the families of seven victims had been notified about the deaths.

“Our first priority is the humane and compassionate removal of the victims of this tragedy,” said Mayor Libby Schaaf. “Secondly we are focused on supporting the families and the loved ones, some who are coming from very far away.

“Our final focus is doing everything that we can to preserve evidence and to conduct the recovery operation in manner that allows us to fully and professionally investigate this incident so that we can get to the bottom of how this happened.”

Schaaf said the city has “activated the criminal investigation team”, which includes staff from the district attorney’s office, but said she would not speculate about causes of the fire or what exactly the city knew about the property, which had a record of complaints and was under investigation before the fire.

“It is far too early for us to have any suspicions about what actually caused this fire,” she said. Police and fire crews will continue “a 24/7 recovery operation”, she added. “The scope of this tragedy is tremendous.”

“This tragedy has hit very close to home,” Kelly said, adding that one of the sheriff’s deputies lost his son in the fire. “We’re finding victims where we least expect them.”

Authorities did not immediately identify any victims, though most were believed to have been in their 20s and 30s. Kelly said that the dead include teenagers, and that some were foreign citizens. Melanie Ditzenerger urged people who are missing loved ones to “preserve any DNA-type of equipment”, such as combs and brushes, in case officials cannot quickly identify a victim.

Earlier on Sunday, an unofficial list of those unaccounted for, compiled by friends and family searching for loved ones, still showed about 30 names. Kelly said that police still do not have a clear estimate about how many people were at the warehouse when the fire broke out.

At a morning press briefing, Kelly said: “In regards to the amount of people that are still missing, yes, it’s a significant number.”

Kelly told reporters teams had worked through the night, with a crane and floodlights, to reach bodies on the second floor of the warehouse. Officials said there were only two exits from the second floor, and no evidence of smoke detectors or sprinklers. Kelly added that there are trailers within the warehouse, where people may have been living, that crews have not been able to search yet.

Firefighters searched the debris “bucket by bucket”, fire battalion chief Melinda Drayton said. “It was quiet, it was heartbreaking,” she added. “This will be a long and arduous process.”

The fire began at about 11.30pm on Friday, blazing for hours and trapping people within the warehouse, which was known locally as “the Ghost Ship”.

Officials said that they did not yet know the cause of the fire, and that there may have been as many as 50 to 100 people in the building when it began.

The warehouse had a history of safety code violations, city records showed, and a former resident told the Guardian that she had reported it to a fire marshal in 2014.

For years, the building had housed art studios and makeshift residences. The former resident said inside was a maze of fire hazards: a staircase made of wooden pallets, old pianos and couches, trinkets, furniture and ramshackle lean-to shelters.

The building lacked permits for housing, work or as a party venue. The city had received complaints about blight and opened an investigation into whether there was illegal construction within the warehouse.

An inspector had failed to gain access before the fire, said Oakland’s building and planning chief, Darin Ranalletti.