You can handle this outside of the criminal justice process for a fair amount of the process.
File a civil suit against "Joe Doe and Does 1 to N, etc." (just like the RIAA) for theft of private property and asking for a judgement ordering the return of the property, etc. In tandem with that, file a request for a subpoena with the ISP to whom this IP belongs for the associated address.
Because the filing of the suit, getting the subpoena, etc., is going to take some time, you should send a letter to the ISP informing them that the subpoena will be coming shortly and that you are informing them of their responsibility to preserve evidence in a pending civil suit.
Once you have the name and address of the party in question, you should do two things:
First, file a formal criminal complaint with the local police and DA concerning stolen property which is being used in their jurisdiction. The original theft may not be their concern, but the receipt of stolen property is there concern.
Second, file a request for in civil court for an order requiring the return of stolen property at address X. Once you have this court order, you can go and get a sheriff (usually at a cost to you) to accompany you to this address and force them to open up and show you it isn't there.
Too often people forget their are parallel legal systems - civil and criminal - in this country and fail to realize that they have control. The RIAA does and takes advantage of it. Why shouldn't you?