I would highly suggest you simply monitor the account and collect evidence if it reveals itself. I know you have good intents, but there are a lot of ways you might cause some unintentional harm here. Unfortunately the user is now aware you suspect them, so if they are up to something illicit they may now attempt to conceal it. If the account does act as if they are setting up fraud of some kind, then I think some more public scrutiny would be warranted.