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A five-alarm blaze that may have been caused by a welding mishap broke out late on Tuesday (local time) at the MB360 apartment complex under construction in the Mission Bay district, said San Francisco Fire Department spokeswoman Mindy Talmadge.
The tower that burned in the two-building, partly built complex had previously stood eight or nine stories tall and was reduced to four stories in the fire, Talmadge said.
"The good thing is that it collapsed in on itself" instead of toward the street, she said.
But 30 apartments in a building across the street had to be evacuated, after heat from the fire blew out that structure's windows and smoke set off its sprinkler system, she said.
Those residents were still not allowed to return home on Wednesday morning, she said. Authorities have set up a perimeter around the burned high-rise tower because of the risk of further collapse, and the structure is a total loss, she said.
About 150 firefighters were deployed against the flames, and hot spots still remained at the site on Wednesday morning.
When construction began last year on the MB360 project, which was to contain 360 residential units in all, the estimated development cost was pegged at $227 million, the San Francisco Chronicle reported.