The five-alarm fire that tore through a Mission Bay 172-unit residential building at Fourth Street and China Basin last month has been ruled accidental.

According to fire officials, a "hot or smoldering object" from the construction sparked the blaze.

"A preliminary San Francisco Fire Department report on the March 11 blaze does not identify the fire-starting object but says it produced enough heat to ignite wood framing between the top floor of the six-story structure and the roof," reports The Chronicle. While KTVU points out, "The fire originated in the southeast section of the building between the top floor and the roof, the report states."

Damage was estimated somewhere around $40 million.