The deadliest and most destructive of last month’s Wine Country wildfires may have been started by electrical equipment not owned or installed by Pacific Gas and Electric Co., the utility said in a legal filing Thursday. The filing states that while California fire investigators are still trying to determine the cause of the Tubbs Fire, which destroyed entire neighborhoods of Santa Rosa, “preliminary investigations suggest that this fire might have been caused by electrical equipment that was owned, installed and maintained by a third party.” A spokesman for the California Department of Forestry and Fire Protection, or Cal Fire, said he did not know the source of PG&E’s information.