An unknown co-worker made a false claim that I was making threats in the workplace. I was brought in to meet the company HR. I firmly explained these allegations are false, and asked who made them as I intended to get a lawyer. The guy told me no lawyer can make him say who it was and that if any other reports come in I will be fired. Recently I was visited by agents of the FBI and Secret Service who told me calls came in that I was making threats to kill the president and was making bombs. These further false allegations resulted in the company HR to call me and inform me I was now fired. How do I file a suit against this person if my former employer refuses to say who they are?