San Francisco construction site fire may have been sparked by welders

San Francisco fire investigators were looking into whether welding work was to blame for a huge blaze that tore through an apartment building construction site, threatening nearby buildings, prompting evacuations and forcing officials to call in about half the city's on-duty firefighters to prevent its spread.

As fire crews doused hotspots in the charred remains of the block-long apartment complex, officials said they narrowly avoided a catastrophe on Wednesday in one of the city's largest fires in recent years. "I think we're very lucky that the fire didn't jump any more," Mayor Ed Lee said.

Two firefighters suffered minor injuries.

Fire officials were looking into preliminary reports that workers were doing torch work at the site just before the fire was reported on Tuesday evening, Fire Chief Joanne Hayes-White said.

"It's too early to determine a cause," Fire Department Battalion Chief Kirk Richardson said.

The fire was fuelled by wood frames and other materials at the site.