Why involve the FBI when you already have the USB? I mean, you lost something and now you found it, what else do you need? If your USB is working just learn how to import it to a safe and secure wallet.
Because of this:
Invalid private key errorHe claims to have purchased this (although that's not how CoinPal worked) and the key is not valid, but I don't see why would the the FBI or even the police would care about this when all he has is a key he
claims to have received from somebody. I also don't see what this has to do with MtGox.