While the details vary from jurisdiction to jurisdictiion, one can file a criminal complaint with the appropriate police agency or prosecutor. Which in this case probably means the federal District Attorney in either the Member's home district or in DC. However, AFAIK, the prosecutor doesn't have to do anything with such a complaint, any more than the do with a criminal charge from the police. They can just ignore it or they can take it to the Grand Jury (or in some cases straight to the Judge).
Wow, thank you. That is enough to help me focus on the next steps.