The FBI isn’t going through tax returns looking for Illegal income; rather if you’re suspected of illegal activity, they WILL go through your tax returns, because if you didn’t report your illegal income that is another law you’ve then broken. — See Al Capone
So. It is both a very bad idea, and a good idea to report your illegal income
A better idea is to not do anything illegal; or at least do the illegal thing offshore and hide the income there. Like a good multinational corporation would.