How do I go about having my previous employer investigated for forgery?

I have knowledge that my previous employer has been forging client signatures and sending the documents to the county. The company I worked for was a tax consulting company that dealt with sewer and sanitation refunds for commercial properties. Due to the clients not willing to give our company the information that was needed in a timely manner, my previous employer would then forge the clients' signature and then send the documents to the corresponding county, which was Los Angeles or Orange County. I would like to know how I can go about having an investigation done on this company. I was recently terminated from this company because they told me that I was too negative and a bad influence on other employees. My last few weeks there, I was ridiculed, harassed, and treated with utmost disrespect by the owners.