I once read that failing to file a tax return if required is a misdemeanor. But, intentionally lying on a filed return is a felony.
Most lawyers will tell you to file the return, report the income, and hope the IRS doesn't figure out the proceeds are from criminal activity.
Even though the IRS doesn't receive a 1099 from most sources of illegal income, they can be alerted to ongoing criminal activity that creates wealth if you have a $50K/yr job, yet live in a $2M mansion, drive 2-3 expensive cars/SUVs, and are generally living well above your means.
There is also the "weird" case where a man was tried for securities fraud. The Supreme Court was petitioned to hear whether or not he could deduct the cost of his lawyers to fight the case in court as a business expense. They ruled unanimously that, since the IRS Code did not state he was not allowed to deduct it, his lawyers were deemed a reasonable cost of doing business. The $22,964.20 in legal fees were deducted from his taxable income. That was a lot of money in 1966!
Since then, Congress passed laws prohibiting specific "costs of doing business" from being used as tax deductions in illegal transactions, such as:
-- Illegal bribes (as opposed to LEGAL bribes?

)
-- illegal Kickbacks
-- Fines and Penalties
-- Expenses for Illegal Drug Sales
In 1927, the Supreme Court ruled that the 5th Amendment protection against self incrimination doesn't shield anyone from reporting ALL income from ANY source to the IRS. That defendant didn't file at all that year. The court said, had he filed and raised the objection to disclose his income under the 5th amendment with his filed forms, he would have had a viable argument. By not filing, he can't come back after being caught and claim 5th Amendment protection for not providing the information at the time he should have filed his return.
I guess it's like, if you get arrested, you have to explicitly state at the time of questioning that you are invoking your 5th Amendment right to not answer questions. If you don't, then remaining silent can be prosecuted as hindering the investigation or obstruction.
The tax laws are a joke. Most of the IRS agents don't even understand them. Call them 3 times, ask the same question of 3 different customer service reps, and you're going to get 2 or 3 different answers. Trust me. I've been there.
This is why they always recommend you get an attorney to deal with the IRS. Trying to do it yourself is like playing "Pin the Tail on the IRS regulation." It's a moving target. If you aren't 100% certain of what the law/code actually says, intends and requires, don't trust the IRS to tell you!