If you WEREN'T investigated, then don't admit that you're mafia. Try to argue your way out, but don't point any fingers either. I think that would be the best way to go about it. If you were investigated, you can do the bomb thing or whatever.

Also, when I looked at the roles page in the link in the main mafia page, they had "Bomb" as a citizen. I don't think there was a "Mafia Bomb." If you want, you can claim that at the end if you weren't investigated.