There's no statute of limitation for tax fraud! The limits of 3 years for small errors and 6 years for large errors. These time limitations ONLY apply to errors. My father was a CPA who never lost a case in tax court. He knew tax code and he told me about the fraud thing being applicable after no time limit whatsoever.
"Is Tax Fraud Civil or Criminal?
A person may be subject to civil or criminal tax fraud. But, unlike most other statutes of limitations which expire after 3-or 6-years, there is no time limit for the IRS to enforce civil fraud penalties."