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Also you’re allowed to plead the fifth on the origin of the income IIRC, though honestly that’s just as likely to get you looked at by the FBI/local constabulary.
On the other hand if you’re making enough money illegally to feel the need to declare it on your taxes (either for ethical reasons or because you don’t want to get done like Al Capone) law enforcement is probably already looking at you.
The IRS doesn’t report it to the FBI because that’s not their job. If you’re already under investigation and the FBI asks, they’ll hand over the info, but they won’t initiate anything, they just want their cut.