California Code, Labor Code - LAB § 7867

The employer shall establish a written procedure for investigating every incident which results in, or, as determined by board criteria, could reasonably have resulted in, a major accident in the workplace.  The procedure shall, at a minimum, require that a written report be prepared and be provided to all employees whose work assignments are within the facility where the incident occurred at the time the incident occurred and shall also include establishing a method for dealing with findings and recommendations.