It’s widely known that Federal Tax Law is complex. This is part of the reason for audits, the civil review of tax filings. How can those reviews turn into a criminal case? There is no simple answer.
One common area is income. Always report all of your income. Numbers that don’t make sense can start audits, and upon further review, it’s up to the discretion of the IRS Auditor where to go. Another contributing factor is statements made during the auditing process. A false statement given is a great way to prolong an audit. This is another area where a qualified Tax Professional will help a Taxpayer. They are a great representative for the Taxpayer to the IRS, and will be able to provide the documents being reviewed.
If the conduct displayed, or the information provided move past a certain threshold for the Auditor, the case may be passed on the IRS Criminal Investigation Division. There is no obligation to inform the Taxpayer of this change in investigation. The audit will probably just pause, and a different part of the IRS will appear.