Pacific Gas & Electric Co. shares plummeted 10.5 percent after state regulators directed the company to preserve any evidence of failed poles, conductors or other equipment that might be connected to Northern California wildfires that killed 32 people and reduced entire neighborhoods to rubble.

The steep one-day fall Friday means the value of the giant utility, or market capitalization, dropped about $3.5 billion, from $33.1 billion to $29.6 billion.

California fire officials are investigating downed power lines and other utility equipment as possible causes of massive wildfires that have destroyed at least 5,700 homes and businesses. The California Public Utilities Commission also directed the utility to tell employees and contractors to preserve emails and other documents related to potential causes of the fires.