As a legal question, there are three main responsible parties. The employer of the person who caused the accident, the companies involved who are not the employer (often called third parties) and the individual.
In legal terms, these break down to:
From a legal context, in California there is no ability to bring a lawsuit against the employer (this is a general rule, there are a few exceptions). You cannot bring a lawsuit against the employer for their role in an accident because it is governed by the exclusive system of Worker’s Compensation. This system prevents lawsuits against employers, but does not provide much compensation if the accident is the fault of the employer.