Officials: Huge San Francisco blaze was accidental

A massive blaze that barreled through an apartment building construction site in San Francisco last month was accidental, not arson, fire investigators said Tuesday.

The five-alarm fire on March 11 in the Mission Bay neighborhood began when a hot or smoldering object ignited wood between the top floor and roof of the six-story building, investigators said in a report.

The fire gutted the building and damaged three other structures. The report estimated damage at about $40 million.

Fire officials said a day after the fire that they were looking into preliminary reports that workers at the block-long site were doing torch work.

A more detailed report that could explain what caused the blaze is expected in the next couple of weeks, fire spokeswoman Mindy Talmadge said.

The fire created a plume of black smoke that was visible for miles and led to the evacuation of nearby buildings. A wall of the burning building collapsed about an hour after the fire began.

City officials said a catastrophe was narrowly avoided in the neighborhood near AT&T Park, home of the Giants.

About 150 firefighters were called in to contain the blaze.