I know my firing was unfair, but was it unlawful?

Question: “I was terminated just before the end of my probationary period. I have my personnel file and there are a number of untruths in it. I had one self-evaluation which was signed and submitted prior to my taking sick leave for a heart catheterization. I received an unsatisfactory evaluation after my return to work. (My position was filled by someone who had formerly been in the office but left and was unhappy in her new position.) Was this an unlawful termination? I am more interested in suing the individuals involved, unless I can possibly get my old position back by suing the organization (or another position at the same location). I appreciate any advice.” — J.F.