To add to the above, you will likely not be able to get a subpoena without involvement of your legal counsel (or law enforcement). In my experience, law enforcement will not get involved unless there are threats of bodily injury or you can demonstrate substantial financial loss.
If you do not pursue the criminal aspect, there are still civil remedies for losses suffered. You will need your legal counsel to get a subpoena then (through the courts). You have a limited time to act as most ISP's only keep IP/username correlations information for 30 days.
Good luck. This is not something that is easily done by non-professionals. You have a tough road ahead.
Certifications: CREA, MCSE: Security, CCNA, Security+, other junk