SAN FRANCISCO — San Francisco fire investigators on Wednesday were looking into whether welding work was to blame for a massive blaze that barreled 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, city officials said they narrowly avoided a catastrophe in an up-and-coming neighborhood near the San Francisco Giants ballpark.
"I think we're very lucky that the fire didn't jump anymore," Mayor Ed Lee said.
The exact cause of the fire — one of the city's largest in recent years — was under investigation. But fire officials were looking into preliminary reports that workers were doing torch work at the site shortly before the fire was reported around 5 p.m. Tuesday, Fire Chief Joanne Hayes-White said.
The five-alarm fire created a plume of black smoke that was visible for miles and led to the evacuation of nearby buildings, as about 150 firefighters were called in to contain it. One of the walls of the burning building collapsed about an hour after the fire began.
"We were hoping that we could contain it as quickly as possible," said firefighter Stephen Maguire, whose crew was among the first to arrive at the scene.
Firefighters had to take a defensive stance because the building was leaning in some areas, he said.
The blaze cracked windows on an apartment building, Strata at Mission Bay, across the street and sent an ember onto the roof of a University of California, San Francisco building in the neighborhood. It also briefly spread to scaffolding at another building under construction, Maguire said.
Two firefighters suffered minor injuries. One of them, Tom Murphy, suffered burns while battling the fire from above, but he didn't realize he was injured for hours, Maguire said.
The fire was fueled by wood frames and other materials at the site, Hayes-White said. Because it was under construction, the building also had no fire-suppression systems.
"Our objective last night, which we were able to achieve, was to contain (the fire) to the building of origin," she said.
Fire officials did not have an exact estimate on the damage on Wednesday, but they said the entire six-story building was considered a loss.
The Strata apartment building remained evacuated on Wednesday.
San Francisco Fire Deputy Chief of Operations Mark Gonzales said exterior sprinklers on that building that went off have to be replaced and the fire protection system restored before residents will be allowed back in. About 30 units suffered damage, fire officials said.
Eli Brill and his wife Justyn Bellsey, both 31, returned to their apartment at Strata late Wednesday morning. Firefighters had helped them retrieve their dog the previous night.
"It was really scary," Bellsey said. "I'm just relieved that we were able to get in."
The burning building was part of a residential development project with 172 units, according to BRE Properties, the building's owner. It was going up in a onetime-industrial area that became a redevelopment district in the 1990s and has since attracted a mix of high-end residential housing and retail businesses. A new University of California, San Francisco hospital is also going up.