Perhaps is the answer that I know you don't want to see. However, many other factors will determine the viability of an action against the government. How this will effect your case is of more concern. Your criminal defense lawyer is in the best position to advise you regarding priorities.
This answer/response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. Please do NOT use this answer/response to say or do anything regarding your situation. BEFORE you say or do anything consult with an experienced Federal and/or state criminal defense attorney in your jurisdiction who will listen to you and your concerns.
The way I read the question you were the victim in the rape case and want to sue the government because the investigator did not turn over evidence to the prosecution. The short answer in my opinion is no. Lawsuits are based upon either Constitution or statutory rights. As a victim you have no constitutional or statutory rights to an investigation.