Investigators Suspect Welding Caused San Francisco Blaze

March 12--SAN FRANCISCO -- Hundreds of Mission Bay apartment dwellers chased from their homes by a huge fire at a building under construction are fortunate to have homes to return to, San Francisco Mayor Ed Lee said Wednesday as firefighters put out hot spots and sought to determine whether welders caused the blaze.

"We're very lucky" that firefighters were able to contain the five-alarm blaze to the building that burned at Fourth and China Basin streets Tuesday, Lee said as firefighters sprayed water on the remains of the six-story, 80-foot-tall structure.

The fire broke out about 5 p.m. and quickly consumed the building, an apartment complex being built by Suffolk Construction Co. that was supposed to open late this year. Smoke that billowed thousands of feet into the air was visible around the Bay Area.

The cause of the fire is not known, though Lee, speaking at a news conference Wednesday morning after touring the site, said construction workers had been welding in the area where the fire is thought to have started.

"It appears that the fire started above ground," Lee said. "They were doing some welding. We don't know what occurred there. I'll be very patient to wait until the investigation is concluded by our Fire Department and by the contractors themselves."

Most of the workers had left for the day before the blaze began, and investigators suspect that the fire could have smoldered for some time before erupting, said Fire Chief Joanne Hayes-White.

The chief said the fire had presented "unique challenges," because the building's sprinklers and other fire-suppression systems had not yet been installed. Half of the Fire Department's on-duty firefighters battled the blaze, she said.

"Our objective last night was to contain the fire to the building of origin," Hayes-White said. "Crews did a phenomenal job."

Lee added, "People will say the city responded well, and our Fire Department deserves some kudos on this."

Blowing embers did briefly ignite a small fire on the roof of a UCSF building about a block away. That fire was quickly put out by building maintenance workers.

Residents of the 190-unit Strata apartment building across Fourth Street from the burning building were evacuated. Lee said heat from the blaze blew out the windows of about 30 units.

Officials did not have a total number of evacuees, but Deputy Fire Chief Mark Gonzales estimated the figure was "in the hundreds."

Fourth Street will remain closed until the area is "deemed safe," Gonzales said.

Two firefighters were hurt. One suffered first- and second-degree burns to his hands and face while on a ladder but continued to fight the fire, authorities said. A battalion chief twisted her ankle. Both were treated and released.

Construction began last year on the building, part of a 360-unit apartment complex being developed by BRE Properties on the west side of Fourth Street. BRE officials issued a statement late Tuesday saying they believed the company's insurance would cover the damages.

The project stands in the midst of at least four other residential construction projects on or near Fourth Street south of Mission Creek. Together, they contain nearly 900 units.

Kale Williams and Henry K. Lee are San Francisco Chronicle staff writers. E-mail: kwilliams@sfchronicle.com and hlee@sfchronicle.com Twitter: @sfkale @henryklee

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