Oakland (United States) (AFP) - Young men and women with glassy eyes waited anxiously in Oakland for firefighters to search a charred building where a blaze may have killed dozens attending a party.

Some paced nervously on the sidewalk beneath an elevated commuter rail line, with voices or sobs drowned out by frequently passing BART trains.

People hugged, paced or smoked nervously. Inside a family center set up in a county building, some 30 more people waited for word about missing friends.

Oakland Mayor Libby Schaaf updated them as morning gave way to afternoon, with not much new to say since firefighters had to back off from searching the building due to fear of collapse.

"We are all just hoping people ditched their phones when they got to the party," said David Marks of San Francisco, who was searching for news of a friend.

Dan Vega worried that his 22-year-old brother might be among the dead. Vega wasn't certain, but thought his brother and a girlfriend may have gone to the party.

Their car was still parked at a BART station near the home the brother shared with his mother, and the young man has given no sign yet.

"It is just not like him," Vega said.

Marks had planned to go to the party after working late, but checked Facebook before setting out and saw news of the deadly blaze.

- 'Ghostship' -

The two-story commercial building dubbed Ghostship was a space where artists lived and worked.

A rickety set of wood plank stairs led to a second floor space used for events. The place was described as being rife with nooks and cluttered with art, statues and other creations.

"And there were tons of organs and pianos," said Marks.

He and others estimated that maybe 20 or 30 people lived in the Ghostship.

Police and fire officials were asking people sharing names, descriptions, clothing and other details about missing friends on social media to get the information to them to help with the difficult task of identifying charred bodies.

Facebook initiated a safety check feature that lets people check-in to let friends know they are safe. News and comment about the tragedy was flooding Twitter with an #oaklandfire hashtag.

Story continues

At the scene, yellow police tape cordoned off the streets around the gray two-story warehouse building, windows blackened and broken, burned furniture piled on the sidewalk below.

The gray building on the edge of a Latino neighborhood bore graffiti and the word Ghostship.

Police were investigating whether what appeared to be bodies in the rubble could in fact be mannequins in the artists' colony.

The roof had caved in to the second floor.

"This is a very rough day for the city of Oakland," Alameda County Sheriff's Department Sergeant Ray Kelly said.

- 'Mass casualty event' -

The county coroner's office was prepared for a "mass casualty event," according to Kelly.

Up to 40 people are feared dead in the tragedy, though the initial death toll stands at nine.

The list of missing included people from outside the US, Kelly said.

"We have a lot of people missing," Kelly said. "We are trying to make sense of all the chaos."

The county coroners office was collecting information about people who might have been at the party and what they were wearing, to help with identifying any bodies found.

"This is going to be a very difficult process for us," Kelly said.

News helicopters hovered overhead as firefighters were preparing drones equipped with thermal imaging capabilities to fly through the building to find "hot spots" that might still be burning inside.