OAKLAND, US - The death toll from a fire at a California warehouse crowded with dance party revelers rose to 36 Monday as fire officials announced they had suspended work because the structure was at risk of collapsing.

Alameda County Deputy Sheriff Tya Modeste said 11 of the 36 victims recovered so far at the site in Oakland have been positively identified. Most of the casualties were in their 20s and 30s but one victims was a 17-year-old boy. Previously, the toll had stood at 33. "We're no closer to finding a cause and we absolutely believe that the number of fire fatalities will increase," Fire Chief Teresa Deloach Reed said.

US President Barack Obama in a statement expressed sadness over the tragedy, the deadliest in California's modern history.

"While we still don't know the full toll of this disaster, we do know that an American community has been devastated, and many people - including young men and women with their whole futures ahead of them - have tragically lost their lives," the statement said.

The fire erupted Friday night in a warehouse occupied by an arts collective known as Oakland Ghost Ship as it hosted a rave attended by between 50 and 100 people.

It spread quickly through the structure, trapping partygoers whose bodies were found scattered through the building's mazelike interior.

Reed said firefighters have reached an area in the back of the building where they believe the fire originated.

The area was cordoned off and work was suspended there late Sunday so that federal investigators could gain access.

But all recovery work was halted shortly after midnight after crews noticed that a parapet atop the building's front exterior wall was leaning in.

"For us firefighters working under a wobbly, potentially collapsing exterior wall is extremely dangerous," said Reed. "We will not put our firefighters in danger at this point."

She said work would resume after structural engineers examine the site and come up with a plan.

Survivors spoke Sunday of the speed with which the fire spread through the warehouse with people banging on windows when they couldn't escape.

Photographer Chris Nechodom, who was at the dance party, said people first thought the smoke was coming from a fog machine.

"And then it got a little thicker," he said. "It all happened within seconds. We started seeing people running around, frantic and screaming 'fire.'"

In a macabre indication of what the fire may have done to the bodies, authorities are asking relatives to preserve hairbrushes and toothbrushes to assist in matching DNA samples.

Although the cause of the fast-moving blaze remains under investigation, Oakland Mayor Libby Schaaf said the district attorney had opened a criminal investigation as a precaution to preserve options as the case unfolds.

"You have to understand that the scope of this tragedy is tremendous," she said. "We have many, many witnesses to interview. We are in the process of doing that."

Some of the victims were from Europe and Asia, and local authorities are working with the State Department to contact foreign governments.

Reed has said the interior of the warehouse was maze-like and was cluttered with furniture, artworks and other items. "There wasn't a real entry or exit path," she said.

City officials acknowledged at the weekend that they had received a number of complaints about the warehouse and that inspectors had been there last month but left when no one answered.

Images published online show artwork, pianos and wooden objects throughout the building, which helps explain why the blaze raced through the structure despite firefighters arriving within minutes.

There were also questions on whether the building was properly equipped with sprinklers or smoke detectors.

Officials said the roof collapsed onto the second floor, which was connected to the ground floor only by a makeshift stairway made of wooden pallets and plywood.

The deadliest nightclub fire in the United States in recent decades occurred in 2003, when pyrotechnic effects by the rock band Great White set off an inferno at The Station nightclub in Rhode Island, killing 100 people.