Not sure... if you personally know that thief you can talk to him and tell him you know and that you think about going to police, however you could say that maybe you don't but you want to know why he did it and what he did with your other accounts he was able to steal as well...
If he cooperates and is sorry you could leave it this way... otherwise as wolf said... oh and if you think the person is more the brutal character don't talk alone to him take a friend with you... just in case. or don't talk to him at all but go to the police right away.
don't know about the lawyer... usually this costs pretty much money and if you haven't some kind of legal costs insurance hmm better the police first that's free and maybe he gets some fear and confess what he did...
Was there any damage done I mean did you lose money or something or "just" some accounts at certain web sides? Since the law might not be very strict he might not even get a punishment but then seeks out revenge on you this should be considered.. you know how low people think... question is how much the damage and if it's worth the hassle I think...